Use WP 6.4 Admin Notices

[fiechtor]

trafficstars

Resolves #1072 .

Updates all notice prints to wp_admin_notice calls. Also updates the WordPress minimum version to due the introduction version of the wp_admin_notice function

[coreymckrill]

@fiechtor Thanks for the PR! Action Scheduler has an L-2 dependency version policy, meaning, in the case of WordPress, that it supports the previous two versions. So right now we are committed to the minimum version of WP being 6.3, but once WP 6.6 comes out, we can certainly consider implementing this PR.

[davidarkemp]

We're on WP 6.8 now, can this be reviewed?