Deprecate Fastlane::Helper::Ios::GitHelper.commit_version_bump

[mokagio]

The helper method relies on the PROJECT_ROOT_FOLDER environment variable, which is no longer meant to be used and has been removed by client apps such as WordPress iOS, wordpress-mobile/WordPress-iOS#21859.

Looking at it today, commit_version_bump seems redundant. It DRYs the message and paths for the committing a version bump, but is it actually necessary to have that logic DRYed in a library when it's possible to achieve the same result with a method, e.g. see wordpress-mobile/WordPress-iOS@f886436, within the project's Fastfile?

I propose we deprecate the helper, or at and schedule it for removal on the next major version.