Make D9 compatible

Attached is a patch that adds the `core_version_requirement` line to the info.yml file, per:
https://www.drupal.org/node/3070687

Unfortunately, it's not applying for me, and not really sure why.
This is the .rej file contents:

***************
*** 1,4 ****
  name: Entity Reference validators
  type: module
  description: 'Provides additional validators for entity reference fields.'
- core: 8.x
--- 1,4 ----
  name: Entity Reference validators
  type: module
  description: 'Provides additional validators for entity reference fields.'
+ core_version_requirement: ^8 || ^9

Perhaps someone else can figure out how to make this patch so it can apply.

Adds patch that applied cleanly to add the missing `core_version_requirement` line.
Note, I had to add the line directly below the `name` property, which is not the ideal location, but it works.

I just noticed after putting the latest dev into my project that it was still giving this in composer.lock

            "require": {
                "drupal/core": "~8"
            },

Which is an issue and will be blocking a D9 upgrade due to that requirement. This should be removed from the module's composer.json file, so that when composer pulls in the project, it can assign the proper values in the composer.lock based on the `core_version_requirement` values present in the info.yml file.

Yup, I already did. See https://www.drupal.org/project/entity_reference_validators/issues/3180478