Move is_utf8_charset() into functions.php

[dmsnell]

Trac ticket: Core-61182

This caused issues in maintenance mode, and it's not warranted to have its own module. This will live alongside _canonical_charset(), it's partner function.

Props: dmsnell, sergeybiryukov, swisspiddy.
Follow-up to: [58148].

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Props dmsnell.

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[dmsnell]

Merged in [58169]
0f57a02