expectDeprecation*(): Document custom extension + events approach for PHPUnit 10

[hellofromtonya]

PHPUnit 10 removed the expect*() methods, including expectDeprecation(), expectDeprecationMessage(), and expectDeprecationMessageMatches().

As PHPUnit Polyfills is forward compatible, these removed expectations will not be polyfilled in 2.x.

As it is possible to polyfill these removed expectations, @jrfnl and I discussed documenting a custom approach in the 2.x README's FAQ.

The custom approach:

• takes inspiration from PHPUnit 11: refer to the Error Handling > Expecting Deprecations docs and the PR that implemented it.
• adds an extension.
• uses the PHPUnit\Event\Test\ events.

[jrfnl]

Closing as duplicate (or close enough) of #186.