Put development/test dependencies into a shared Gemfile

[st0012]

With 6 gems under this repo, it's been a headache to update development tooling for them with consistency. I think using a shared Gemfile to host all development (e.g. debug, irb) and test (e.g. rspec) dependencies will make this easier.

It also fixes the master CI as debug now requires Ruby 2.7+.

#skip-changelog

[codecov]

Codecov Report

Merging #2192 (0a9ef63) into master (f3e94e0) will increase coverage by 2.62%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2192      +/-   ##
==========================================
+ Coverage   64.06%   66.68%   +2.62%     
==========================================
  Files          84       98      +14     
  Lines        3339     3659     +320     
==========================================
+ Hits         2139     2440     +301     
- Misses       1200     1219      +19     

Components	Coverage Δ
sentry-ruby	55.95% <ø> (ø)
sentry-rails	94.98% <ø> (+0.01%)	⬆️
sentry-sidekiq	94.53% <ø> (∅)
sentry-resque	92.06% <ø> (∅)
sentry-delayed_job	94.44% <ø> (∅)
sentry-opentelemetry	100.00% <ø> (ø)

see 14 files with indirect coverage changes