Accessibility: Security: Improves the accessible audible messages for Plugins...

Accessibility: Security: Improves the accessible audible messages for Plugins & Themes Auto-Updates.

- changes the politeness level of the two error messages introduced in [47835] to `assertive`
- remove unnecessary `polite` parameters as that's the default value

See #50512, #50052.


Built from https://develop.svn.wordpress.org/trunk@48479


git-svn-id: https://core.svn.wordpress.org/trunk@48248 1a063a9b-81f0-0310-95a4-ce76da25c4cd