Make module D9 compatible

Thanks, but actually there is no need to limit the module only to 8.7.7 and D9.

core_version_requirement: ^8 || ^9 is sufficient.

I can change that on commit if that is fine with you.

as this discussed in Slack right now this should further limit D8 to >= 8.3.

We could solve it similar to #3096615: Make Drupal 9 ready and remove deprecated code usages;

+++ b/prefetch_cache.info.yml
@@ -2,5 +2,7 @@ name: Prefetch Cache
 core: 8.x
+core_version_requirement: ^8 || ^9
 dependencies:
+  - drupal:system (>= 8.3)

Will change it on commit.

Committed and pushed to 8.x-1.x. Thank you!