ruby: Use two spaces per indentation level (#11869)

Hello, this pull request changes the indentation level for Ruby language
from 2 spaces to the most used setting in the Ruby world: 2 spaces per
indentation level.
This setting is mentioned in the [Ruby style guide from the Rubocop
(Ruby linter and formatter)
team](https://rubystyle.guide/#spaces-indentation) and/or in another
popular Rubocop configuration tool -
[`standardrb`](https://github.com/standardrb/standard/blob/main/config/base.yml#L233)
Thanks!

Release Notes:

- N/A

extensions/ruby/languages/ruby/config.toml

@@ -38,3 +38,4 @@ brackets = [
     ] },
 ]
 collapsed_placeholder = "# ..."
+tab_size = 2