Fix PGP Signature change warning

[sethwv]

This pull request updates the validation workflow for plugin changes, with a particular focus on handling updates to the public key used for plugin signature verification. The main improvements ensure that changes to the public key are properly detected and that the reporting step is always run when the key changes, regardless of whether plugin validation is skipped.

Key workflow and script changes:

Public key change detection and reporting:

• The detect-changes.sh script now checks if the public key file (.github/scripts/keys/dispatcharr-plugins.pub) has changed, and sets a new output variable pub_key_changed accordingly.
• The workflow condition for the report job in .github/workflows/validate-plugin.yml is updated to always run the report step if the public key has changed, even if plugin validation is skipped.

Reporting improvements:

• The report script's message is clarified to indicate that existing embedded signatures in manifest.json files (rather than just .sig files) will be regenerated on the next publish, reflecting the actual files affected by a key change.