Fix missing passing grade

[TamaroWalter]

🔀 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:

This PR fixes #251.

Part of the completion conditions for activity completions are conditions regarding a users grade. Teacher can depend the completion on a passing grade.

What was the problem?
Moodleoverflow did not have a passing grade setting. As the moodle core accepts passing grades as completion conditions, a teacher could set the passing grade as condition but the mod_form-validation returned the error that a passing grade needs to be set, although there was no possibility to set it.

What was the solution?
A new field is added to mod_form in the grades section where a teacher can set a passing grade. This field is recognized automatically by the mod_form and handled correctly. A new field is added to the database as well as the setting needs to be saved if the teacher wants to change it.

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📋 Checklist

Please confirm the following (check all that apply):

• 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.

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🔍 Related Issues

• Related to No way to set passing grade for completion #251

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