Allow using the latest version of mocha even when using Ruby < 2.7
In #37883, we updated the Gemfile to restrict the version of mocha to avoid the following error that occurs when using mocha 2.0.0 with Ruby < 2.7.
cannot load such file -- ruby2_keywords (LoadError)
But we can now remove the restriction because the above problem was fixed in mocha 2.0.1 (see https://github.com/freerange/mocha/blob/v2.0.1/RELEASE.md#external-changes).
Patches are attached.