Add new lifecycle step: deletebackup

[mikeJKU]

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

• Step deletes backups from database and file system.
• Step has one setting: "Delete all backups that are older than x days". You can also set it to -1, which will automatically delete ALL backups.
• Output in scheduled task logs: Record id: {recordid} (deleted: 0, 1 or -1), file: {full path to backup file} (deleted: 0 or 1)
  Explanation: 1 = successfully deleted; 0 = could not be deleted; -1 = DB: Entry was not deleted, because backup file could not be deleted, Backup file: Should not happen unless an error occurs in the plugin

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📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that
  should not appear on the productive branch.
• Code only uses language strings instead of hard-coded strings.
• If there are changes in the database: I updated/created the necessary upgrade steps in db/upgrade.php and
  updated the version.php.
• If there are changes in javascript: I build new .min files with the grunt amd command.
• If it is a Moodle update PR: I read the release notes, updated the version.php and the CHANGES.md.
  I ran all tests thoroughly checking for errors. I checked if bootstrap had any changes/deprecations that require
  changes in the plugins UI.

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🔍 Related Issues

• Related to Delete tool_lifecycle backups #44

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🧾📸🌐 Additional Information (like screenshots, documentation, links, etc.)

Screenshot of scheduled task log of step deletebackup:

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