Release 15.0.0 into trunk

[mokagio]

Releasing new version 15.0.0.

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

- The `ios_build_preflight` and `android_build_preflight` actions have been removed. [#773]
- `EnvManager` now populates the process `ENV` from the loaded `.env` file by default [#723].

### New Features

- `get_prs_between_tags` now accepts a `fail_on_error:` parameter. [#772]

[Copilot]

Pull request overview

This PR performs the standard release cut for Release Toolkit 15.0.0 by bumping the gem version and moving the pending ## Trunk changelog entries into a new ## 15.0.0 section, aligning the repository state with the new major release.

Changes:

• Bump the plugin version to 15.0.0 in the version constant.
• Update Gemfile.lock to reflect the new gem version.
• Finalize CHANGELOG.md for the release by moving Trunk entries into ## 15.0.0 and resetting Trunk placeholders to _None_.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/fastlane/plugin/wpmreleasetoolkit/version.rb	Bumps the gem’s VERSION constant to 15.0.0.
Gemfile.lock	Updates the local path gem version entry to 15.0.0.
CHANGELOG.md	Creates the ## 15.0.0 section with the release notes and clears Trunk sections back to _None_.

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[AliSoftware]

@mokagio Can I ask you to check on this small follow-up in GutenbergKit (which was waiting on this to DRY some logic instead of duplicating it in its Fastfile) once this ships?

[mokagio]

Sure thing! As a matter of fact, that's the reason I created this new release.