[BCB] Remove GenericTypeVariableResolver

Author: ondrejmirtes

UPGRADING.md

@@ -243,3 +243,4 @@ Instead of `PHPStanTestCase::createBroker()`, call `PHPStanTestCase::createRefle
 * Remove `AnalysisResult::getInternalErrors()`, use `getInternalErrorObjects()` instead
 * Remove `ConstantReflection::getValue()`, use `getValueExpr()` instead. To get `Type` from `Expr`, use `Scope::getType()` or `InitializerExprTypeResolver::getType()`
 * Remove `PropertyTag::getType()`, use `getReadableType()` / `getWritableType()` instead
+* Remove `GenericTypeVariableResolver`, use [`Type::getTemplateType()`](https://apiref.phpstan.org/2.0.x/PHPStan.Type.Type.html#_getTemplateType) instead