Upgrade to Ruby 2.7.6

[filbranden]

Ruby 2.7.6 is the remaining Ruby 2.x version officially supported in Heroku, so let's move to that.

Currently looking into upgrading to Ruby 3.x, but that breaks some tests so I need to further look into this.

Edited Gemfile to update the Ruby version and ran bundle update --bundler. Also updated the .ruby-version file for rbenv.

Tested: pushed to staging and production, confirmed it works as expected on both.

% heroku run -a vimgolf rails console
Running rails console on ⬢ vimgolf... up, run.6629 (Hobby)
Loading production environment (Rails 5.2.6)
irb(main):001:0> RUBY_VERSION
=> "2.7.6"

[codecov-commenter]

Codecov Report

Merging #357 (9448a0e) into master (fae7026) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #357   +/-   ##
=======================================
  Coverage   92.89%   92.89%           
=======================================
  Files          27       27           
  Lines         802      802           
=======================================
  Hits          745      745           
  Misses         57       57           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.