Fix PHP 8.2 deprecations

[mdio]

Several things happened here in order to support current PHP versions:

• Upgrade PHPUnit to 9 + fix test signature and bcmath errors with passing strings like "two"
• Replace utf8_decode with mb_convert_encoding (as it's deprecated in PHP 8.2)
• I then removed the code that was running when ext-mbstring wasn't available as it's a requirement now anyway.

[mdio]

@DaveChild could you please take a look at this PR? We have this running in production just fine for over a year now.

Let me know if there's anything I can do to improve the PR.

[JoshHighland]

@DaveChild Can we get a merge on this?

[linh-ngophuochoai]

@DaveChild would love to have this PR merged.