Fix PHP 8.0 ValueError calling mb_convert_encoding() with an invalid encoding

Summary:
Per https://www.php.net/manual/en/function.mb-convert-encoding.php, as of PHP 8.0.0, a ValueError is thrown if the value of to_encoding or from_encoding is an invalid encoding but a ValueError is not suppressed by the stfu operator ("@").

Origin of the function:

https://secure.phabricator.com/rPHU72ad8fd0f05b0d84f7d8efd7db62ad0b3ba4431f

Premising that Arcanist elevates warnings to exception, now we just try-catch.

Closes T15423

Test Plan:
On /diffusion/edit/1/page/encoding/,

• enter a valid encoding, such as "7bit", successfully changed encoding
• enter a valid encoding with random capitalization, such as "7biT", successfully changed encoding
• enter a valid alias encoding, such as "ISO-10646-UCS-2", successfully changed encoding
• enter a valid alias encoding with random capitalization, such as "isO-10646-uCS-2", successfully changed encoding
• enter an invalid encoding, such as "whatever", get error message "Repository encoding "whatever" is not valid: String conversion from encoding 'UTF-8' to encoding 'whatever' failed: mb_convert_encoding(): Argument #2 ($to_encoding) must be a valid encoding, "whatever" given"

In any case, no exception is shown anymore.

Reviewers: O1 Blessed Committers, valerio.bozzolan, speck

Reviewed By: O1 Blessed Committers, valerio.bozzolan, speck

Subscribers: 0, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15423

Differential Revision: https://we.phorge.it/D25249