Use RSpec 3.12

[gmcgibbon]

Fixes ERB warnings when running tests:

> bundle exec rspec
/home/runner/work/rack-mini-profiler/rack-mini-profiler/vendor/bundle/ruby/3.1.0/gems/rspec-core-3.[6](https://github.com/MiniProfiler/rack-mini-profiler/actions/runs/4128280759/jobs/7132528979#step:6:7).0/lib/rspec/core/configuration_options.rb:1[7](https://github.com/MiniProfiler/rack-mini-profiler/actions/runs/4128280759/jobs/7132528979#step:6:8)1: warning: Passing safe_level with the 2nd argument of ERB.new is deprecated. Do not use it, and specify other arguments as keyword arguments.
/home/runner/work/rack-mini-profiler/rack-mini-profiler/vendor/bundle/ruby/3.1.0/gems/rspec-core-3.6.0/lib/rspec/core/configuration_options.rb:171: warning: Passing trim_mode with the 3rd argument of ERB.new is deprecated. Use keyword argument like ERB.new(str, trim_mode: ...) instead.
...

[codecov-commenter]

Codecov Report

Merging #554 (3a72192) into master (edbcad3) will not change coverage. The diff coverage is n/a.

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@@           Coverage Diff           @@
##           master     #554   +/-   ##
=======================================
  Coverage   87.39%   87.39%           
=======================================
  Files          18       18           
  Lines        1269     1269           
=======================================
  Hits         1109     1109           
  Misses        160      160           

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[nateberkopec]

Rebase to show CI is green and let's 🚢

[nateberkopec]

Merged in f5abf2d571449801a768b0064057c1af688e6695