[#393] Drupal 9 deprecations fixes

[arlina-espinoza]

Makes apigee_edge module and submodules compatible with Drupal 9, as checked with the deprecation checker tool mglaman/drupal-check.

[mxr576]

@cnovak Is this PR going to create a new major version form the module? There are quite some changes on the public API or the module (like new menthods in interfaces, changed return types) that could break downstream projects that were using these.
(Removing deprecated code usages (which is a 👍 ) does not necessary require adding breaking changes to the codebase, see Drupal core.)

What is the strategy for supporting Drupal 8.x and Drupal 9.x when Drupal 9.0.0 it is out?

[arlina-espinoza]

@mxr576 We have discussed creating a new 2.x branch that supports both D8 and D9. However, I have updated the PR and I think there are no breaking changes any more.
Other notes:

• The Apigee Edge Actions (rules integration) and submodules are not marked compatible with D9, as rules doesn't have a compatible release yet.

Testing
To test against the latest D9 beta, the key module needs this patch (https://www.drupal.org/project/key/issues/3116139). However, applying it using composer patches will still fail because composer says there is no available package of drupal/key compatible for D9. I forked the key module repo to make testing easier, and added it to the repositories section of the composer.json. In all. I'm using the following composer.json template to install and run D9 (it includes some fixes for other dev dependencies that were failing):
https://gist.github.com/arlina-espinoza/70515063318d23d88aabed2b18aac6c8

[kscheirer]

Instead of strtolower() I suggest using mb_strtolower() instead, which is multibyte safe. The change record is here: https://www.drupal.org/node/2850048.

[arlina-espinoza]

Instead of strtolower() I suggest using mb_strtolower() instead, which is multibyte safe. The change record is here: https://www.drupal.org/node/2850048.

Good catch @kscheirer ! I have updated the PR to use multibyte functions where user input is processed. 👍

[shadcn]

This looks good. Thanks @arlina-espinoza

Re. the singular label/lowercase deprecation, I think we should make the change so that label_singular annotation on the entity type is lowercase. This is how core does it: https://git.drupalcode.org/project/drupal/-/commit/13822906e5d7704a0df00dbd930b09a71f4b1972

See also \Drupal\Core\Entity\EntityTypeInterface::getSingularLabel

/**
   * Gets the indefinite singular form of the name of the entity type.
   *
   * This should return the human-readable name for a single instance of
   * the entity type. For example: "opportunity" (with the plural as
   * "opportunities"), "child" (with the plural as "children"), or "content
   * item" (with the plural as "content items").
   *
   * Think of it as an "in a full sentence, this is what we call this" label. As
   * a consequence, the English version is lowercase.
   *
   * @return string|\Drupal\Core\StringTranslation\TranslatableMarkup
   *   The singular label.
   */
  public function getSingularLabel();

[shadcn]

@arlina-espinoza Thanks. LGTM.