doc: fix default rails version on CONTRIBUTING.md

[sanfrecce-osaka]

Description

Now, default rails version in sentry-rails is 8.0.

cf. https://github.com/getsentry/sentry-ruby/blob/5.24.0/sentry-rails/Gemfile#L20

But written on CONTRIBUTING.md is 7.0. So I fixed it.

#skip-changelog

[solnic]

Thank you! <3

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.90%. Comparing base (90d5a3a) to head (fd8203c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2636      +/-   ##
==========================================
- Coverage   96.13%   94.90%   -1.24%     
==========================================
  Files         133      133              
  Lines        5123     5100      -23     
==========================================
- Hits         4925     4840      -85     
- Misses        198      260      +62     

Components	Coverage Δ
sentry-ruby	94.92% <ø> (-2.48%)	⬇️
sentry-rails	94.31% <ø> (-0.60%)	⬇️
sentry-sidekiq	93.84% <ø> (+13.76%)	⬆️
sentry-resque	94.44% <ø> (ø)
sentry-delayed_job	94.68% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (+0.68%)	⬆️

see 41 files with indirect coverage changes

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