Backport PR #13784 to 7.17: Deps: pin sinatra due (2.2.0) incompatibilities

[kares]

Backport PR #13784 to 7.17 branch. Original message:

Dependency lock.

Release notes

[rn:skip]

What does this PR do?

Locks sinatra dependency due incompatibilities.
Sinatra is used for LS's HTTP API.

This concerns LS 8.0.0 whenever a plugin update is triggered (which updates sinatra).
Report at: #13777

Sample failure (from CI while using sinatra 2.2.0):

ArgumentError: wrong number of arguments (given 3, expected 1..2)
  # /opt/logstash/logstash-core/lib/logstash/api/modules/base.rb:43:in `initialize'
  # /opt/logstash/vendor/bundle/jruby/2.5.0/gems/sinatra-2.2.0/lib/sinatra/base.rb:1537:in `new'
  # /opt/logstash/logstash-core/lib/logstash/api/rack_app.rb:113:in `block in app'
  # /opt/logstash/vendor/bundle/jruby/2.5.0/gems/rack-2.2.3/lib/rack/builder.rb:125:in `initialize'
  # /opt/logstash/logstash-core/lib/logstash/api/rack_app.rb:101:in `app'
  # /opt/logstash/logstash-core/lib/logstash/webserver.rb:123:in `initialize'
  # /opt/logstash/logstash-core/lib/logstash/webserver.rb:74:in `from_settings'
  # /opt/logstash/logstash-core/lib/logstash/agent.rb:69:in `initialize'

LS (>=) 8.0.1 should lock the version until the issue is resolved ...

[robbavey]

LGTM, assuming green