Nitpick follow-up to update_fix_compatibility() fix

Addressed all the items in #2, except #2.5 which was already fixed. I also converted the anonymous functions to arrow functions at the same time.

Can we update the summary.

Thanks!

Transcribed the proposed resolution from #2

Thanks! Seems like a good clean up to me

Appears to need a rebase.

Rebased and swapped !in_array($extension_name, array_keys(DRUPAL_CORE_REMOVED_THEME_LIST), TRUE) to !array_key_exists($extension_name, DRUPAL_CORE_REMOVED_THEME_LIST).

Wondering if it makes sense to move DRUPAL_CORE_REMOVED_MODULE_LIST and DRUPAL_CORE_REMOVED_THEME_LIST constants to this class while we're at it. It's not part of the original scope, but not sure that matters. The only place they are used is in SystemRequirements and UpdateScriptTest.

Seems like a good rebase