[TASK] Deprecate passing Rule to RuleSet::getRules()

Relates to #1247.

Author: JakeQZ

CHANGELOG.md

@@ -46,6 +46,9 @@ Please also have a look at our
 
 ### Deprecated
 
+- Passing a `Rule` to `RuleSet::getRules()` is deprecated,
+  affecting the implementing classes `AtRuleSet` and `DeclarationBlock`
+  (call `getRules($rule->getRule())` instead) (#1248)
 - Passing a string as the first argument to `getAllValues()` is deprecated;
   the search pattern should now be passed as the second argument (#1241)
 - Passing a Boolean as the second argument to `getAllValues()` is deprecated;

src/RuleSet/RuleSet.php

@@ -135,7 +135,8 @@ public function addRule(Rule $ruleToAdd, ?Rule $sibling = null): void
      *        Pattern to search for. If null, returns all rules.
      *        If the pattern ends with a dash, all rules starting with the pattern are returned
      *        as well as one matching the pattern with the dash excluded.
-     *        Passing a `Rule` behaves like calling `getRules($rule->getRule())`.
+     *        Passing a `Rule` for this parameter is deprecated in version 8.9.0, and will not work from v9.0.
+     *        Call `getRules($rule->getRule())` instead.
      *
      * @return array<int<0, max>, Rule>
      */