[depre] Remove strict-booleans set deprecated since 2025-10

[TomasVotruba]

The strict-booleans set was deprecated on 2025-10-21 in #7523 - 9 months ago. Since that cleanup it holds exactly one rule:

// config/set/strict-booleans.php
$rectorConfig->rules([DisallowedEmptyRuleFixerRector::class]);

That rule is already registered in CodeQualityLevel, so the set adds nothing the code-quality set does not already give you.

The strictBooleans flag of withPreparedSets() was already inert - it printed a deprecation warning and was never part of $setMap, so it never applied any rule:

-        if ($strictBooleans) {
-            $message = 'The "strictBooleans" set is deprecated as mostly risky and not practical. ...';
-
-            $symfonyStyle = new SymfonyStyle(new ArgvInput(), new ConsoleOutput());
-            $symfonyStyle->warning($message);
-        }

Removed here:

• config/set/strict-booleans.php
• SetList::STRICT_BOOLEANS
• the strictBooleans param and its warning block in RectorConfigBuilder
• the Strict Booleans entry in CoreSetProvider

DisallowedEmptyRuleFixerRector itself stays, reachable through the code-quality set.

Note this is a BC break for configs still naming SetList::STRICT_BOOLEANS or passing strictBooleans: true - they now fail instead of warning.

ECS, PHPStan, Rector and PHPUnit (5472 tests) pass.