solargraph: needs ruby 2.7+

```
  ERROR:  Error installing solargraph-0.49.0.gem:
  	The last version of rubocop (~> 1.38) to support your Ruby & RubyGems was 1.50.2. Try installing it with `gem install rubocop -v 1.50.2` and then running the current command again
  	rubocop requires Ruby version >= 2.7.0. The current ruby version is 2.6.10.210.
```

relates to Homebrew#142161 (comment)

Signed-off-by: Rui Chen <rui@chenrui.dev>

Formula/s/solargraph.rb

@@ -17,7 +17,8 @@ class Solargraph < Formula
     sha256 cellar: :any_skip_relocation, x86_64_linux:   "faeca34a6d2716350ec139ddaa22993fc65ce2cd01dddb8a3d8098a21be5c358"
   end
 
-  uses_from_macos "ruby", since: :catalina
+  depends_on "ruby" # Requires >= Ruby 2.7
+
   uses_from_macos "xz"
 
   def install