Warn in advance of removing support for the match parameter on URL callbacks

[dgw]

The match parameter on URL callbacks has been obsolete since Sopel 7.1 (#1904), and is due to be removed in Sopel 9.0.

However, plugins whose authors (or users) don't read release notes might not notice that they're still relying on an outdated parameter. The code to handle this parameter should emit a deprecation warning starting in Sopel 8.1.0 to catch as many stragglers as possible and bring them into the new world where trigger is always The Only Way to access rule matches.

(At the same time, we should sweep built-in/first-party plugins for use of the deprecated parameter and update any found.)