Release 12.0.0 into trunk

[iangmaia]

Releasing new version 12.0.0.

What's Next

PR Author: Be sure to create and publish a GitHub Release pointing to trunk once this PR gets merged,
copy/pasting the following text as the GitHub Release's description:

### Breaking Changes

- Removed `build_gradle_path` parameter from `android_current_branch_is_hotfix`. [#579]
- Deleted `Fastlane::Helper::Android::GitHelper` and `Fastlane::Helper::Ios::GitHelper`. [#579]
- Renamed `create_release` to `create_github_release` for consistency. [#585, #588]
- Deleted the following deprecated actions: [#577, #579, #586]
    - `android_betabuild_prechecks`
    - `android_build_prechecks`
    - `android_bump_version_beta`
    - `android_bump_version_final_release`
    - `android_bump_version_hotfix`
    - `android_bump_version_release`
    - `android_codefreeze_prechecks`
    - `android_completecodefreeze_prechecks`
    - `android_finalize_prechecks`
    - `android_get_alpha_version`
    - `android_get_app_version`
    - `android_get_release_version`
    - `android_hotfix_prechecks`
    - `android_tag_build`
    - `ios_betabuild_prechecks`
    - `ios_build_prechecks`
    - `ios_bump_version_beta`
    - `ios_bump_version_hotfix`
    - `ios_bump_version_release`
    - `ios_codefreeze_prechecks`
    - `ios_completecodefreeze_prechecks`
    - `ios_current_branch_is_hotfix`
    - `ios_finalize_prechecks`
    - `ios_get_app_version`
    - `ios_get_build_number`
    - `ios_get_build_version`
    - `ios_hotfix_prechecks`
    - `ios_tag_build`
    - `ios_validate_ci_build`

### New Features

- Added the `publish_github_release` action to publish existing GitHub draft releases. [#585]

### Bug Fixes

- Fix `create_release_backmerge_pull_request_action` error when creating a backmerge to a branch not yet fetched locally. [#587]

### Internal Changes

- Update gems used internally & remove upper bound on `nokogiri` from the `.gemspec`. [#583]
- Updated our internal Ruby dependencies. [#582]