Update the `rspec` GitHub Action to properly execute

Description

Update .github/workflows/rspec.yml so that the rspec GitHub action executes properly, and pull requests can be merged with greater confidence.

Motivation and Context

The rspec action is failing in the master branch with an error message instructing to migrate to ruby/setup-ruby. That version of setup-ruby automatically installs a bundler version compatible with the specified Ruby version, so it's no longer necessary to install bundler directly. Also, rspec is included as a development dependency within the gemspec, so it's not necessary to install that directly either.

How Has This Been Tested?

The rspec action now runs successfully: https://github.com/samg/timetrap/pull/218/checks

Screenshots (if appropriate):

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[x] My code follows the code style of this project.
[ ] I have updated the documentation accordingly.
[ ] I have added tests to cover my changes.
[x] All new and existing tests passed.

samg / timetrap