Deprecate Ruby 2.6 & Rails 5.2 officially

[brunoocasali]

• Deprecate Ruby 2.6 and Ruby on Rails 5.2 (both are EOL since more than 1 year ago).
• Update meilisearch to 0.25.0

The minimum required version constraint in the gemspec is still 2.6 and will remain like that until we adopt a breaking feature for that version.
This way, we let the use decide when they will be able to move on!

Introduce a change from meilisearch-ruby 0.25.0. That will trigger a warning message when used with camelCase attributes.
Check the release changelog of meilisearch-ruby: https://github.com/meilisearch/meilisearch-ruby/releases/tag/v0.25.0

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (86d6aba) 88.81% compared to head (9854d74) 88.81%.

❗ Current head 9854d74 differs from pull request most recent head 450dc36. Consider uploading reports for the commit 450dc36 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #275   +/-   ##
=======================================
  Coverage   88.81%   88.81%           
=======================================
  Files          10       10           
  Lines         644      644           
=======================================
  Hits          572      572           
  Misses         72       72           

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