Revert "Upgrade to ruby 2.7.0 💎"

[sue445]

Reverts #289

FrozenError: can't modify frozen String: ""
File "/app/vendor/bundle/ruby/2.7.0/gems/http-3.3.0/lib/http/response/body.rb" line 52 in force_encoding
File "/app/vendor/bundle/ruby/2.7.0/gems/http-3.3.0/lib/http/response/body.rb" line 52 in to_s
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/forwardable.rb" line 229 in empty?
File "/app/vendor/bundle/ruby/2.7.0/gems/mastodon-api-2.0.0/lib/mastodon/rest/request.rb" line 26 in perform
File "/app/vendor/bundle/ruby/2.7.0/gems/mastodon-api-2.0.0/lib/mastodon/rest/utils.rb" line 10 in perform_request
File "/app/vendor/bundle/ruby/2.7.0/gems/mastodon-api-2.0.0/lib/mastodon/rest/utils.rb" line 18 in perform_request_with_object
File "/app/vendor/bundle/ruby/2.7.0/gems/mastodon-api-2.0.0/lib/mastodon/rest/media.rb" line 17 in upload_media
File "/app/lib/twittodon/mastodon.rb" line 33 in upload_media_url
File "/app/lib/twittodon/service.rb" line 112 in block in upload_twitter_medias_to_mastodon
File "/app/lib/twittodon/service.rb" line 111 in each
File "/app/lib/twittodon/service.rb" line 111 in each_with_object
File "/app/lib/twittodon/service.rb" line 111 in upload_twitter_medias_to_mastodon
File "/app/lib/twittodon/service.rb" line 72 in toot_tweet
File "/app/lib/twittodon/service.rb" line 65 in block (2 levels) in perform
File "/app/lib/twittodon/service.rb" line 138 in capture_error
File "/app/lib/twittodon/service.rb" line 64 in block in perform
File "/app/lib/twittodon/service.rb" line 63 in each
File "/app/lib/twittodon/service.rb" line 63 in perform
File "/app/Rakefile" line 34 in block in <top (required)>
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 281 in block in execute
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 281 in each
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 281 in execute
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 219 in block in invoke_with_call_chain
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 199 in synchronize
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 199 in invoke_with_call_chain
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/task.rb" line 188 in invoke
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 160 in invoke_task
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 116 in block (2 levels) in top_level
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 116 in each
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 116 in block in top_level
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 125 in run_with_threads
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 110 in top_level
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 83 in block in run
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 186 in standard_exception_handling
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/lib/rake/application.rb" line 80 in run
File "/app/vendor/bundle/ruby/2.7.0/gems/rake-13.0.1/exe/rake" line 27 in <top (required)>
File "/app/vendor/bundle/ruby/2.7.0/bin/rake" line 23 in load
File "/app/vendor/bundle/ruby/2.7.0/bin/rake" line 23 in <top (required)>
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/cli/exec.rb" line 63 in load
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/cli/exec.rb" line 63 in kernel_load
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/cli/exec.rb" line 28 in run
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/cli.rb" line 476 in exec
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/vendor/thor/lib/thor/command.rb" line 27 in run
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/vendor/thor/lib/thor/invocation.rb" line 127 in invoke_command
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/vendor/thor/lib/thor.rb" line 399 in dispatch
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/cli.rb" line 30 in dispatch
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/vendor/thor/lib/thor/base.rb" line 476 in start
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/cli.rb" line 24 in start
File "/app/vendor/ruby-2.7.0/lib/ruby/gems/2.7.0/gems/bundler-2.1.2/libexec/bundle" line 46 in block in <top (required)>
File "/app/vendor/ruby-2.7.0/lib/ruby/2.7.0/bundler/friendly_errors.rb" line 123 in with_friendly_errors
File "/app/vendor/ruby-2.7.0/lib/ruby/gems/2.7.0/gems/bundler-2.1.2/libexec/bundle" line 34 in <top (required)>
File "/app/vendor/bundle/bin/bundle" line 113 in load
File "/app/vendor/bundle/bin/bundle" line 113 in <main>

[sue445]

c.f. httprb/http#582