Block automerge itself for semver major

[Eomm]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Refs:

• major: remove backing web app #107 (comment)
• Remove backing web app #103

When we release a major version of this GHA, dependabot will open a PR against this GHA users.
The github-action-merge-dependabot will merge the PR updating itself assuming users has the default target: any configuration.

This must not happen: a major bump of this GHA must be done manually by the user because a major release (could) mean a new GHA syntax.

The github-action-merge-dependabot cannot upgrade itself regardless the target filter.

In this case, we should:

• if the api_url is the default value:
  • do the normal flow: the external application will take care of rejecting the PR after landing (dismissing app) Block automerge itself for semver major dependabot-merge-action-app#105
• if the api_url is not the default value:
  • set the GHA step as FAILED with a how to upgrade message
  • log the same message as a warning
  • (optional if possible) comment the PR with the same message