Resolve #3147477 "Automated drupal 9"

Cumulative Drupal 9 Compatibility Patches.

This MR combines all the Drupal 9 compatibility patches from

https://www.drupal.org/project/field_group_ajaxified_multipage/issues/3147477#comment-13670595

plus additional patches for Drupal 9 compatibility and coding standards, including:

• using \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead of t()
• declaring core_version_requirement in *.info.yml files.

It is useful to have an issue fork for Drupal 9 compatibility, rather than simply a patch, because:

In short, you can't use a patch to tell Composer that a Drupal 8 module is compatible with Drupal 9.

Source: https://www.mediacurrent.com/blog/how-fix-catch-22-problem-drupal-9-fixes-composer/

Adding Issue Fork as a Composer repository:

To use this issue fork as a transitional D9-compatible package in your project, modify your composer.json:

    "repositories": {
        "drupal/field_group_ajaxified_multipage": {
            "type": "vcs",
            "url": "https://git.drupalcode.org/issue/field_group_ajaxified_multipage-3147477"
        },
        "drupal": {
          "type": "composer",
          "url": "https://packages.drupal.org/8",
          "exclude": ["drupal/field_group_ajaxified_multipage"]
        },

Requiring the patched package from the Issue Fork:

Execute the following command in a terminal:

$ composer require drupal/field_group_ajaxified_multipage:dev-3147477-automated-drupal-9

or manually add the requirement to composer.json:

    "require": {
        "drupal/field_group_ajaxified_multipage": "dev-3147477-automated-drupal-9",

Credit due, mainly, to Benji Fisher via: https://www.drupal.org/project/views_url_alias/issues/3142138#comment-14177409

NOTE:

Instructions above tested (with Drupal 8.9.18) and patched module now passes Upgrade Status tests.

Instructions above tested (with Drupal 9.2.5) and D9-compatible module installs via Issue Fork. 👍

Closes #3147477