Extra Generators: Always warn

Issue a warning message rather than a deprecation diagnostic when 'extra
generators' are used. This prevents `-Wno-author` suppressing the
message, which really isn't a diagnostic for authors as it relates to
how CMake was invoked.

Source/cmGlobalGenerator.cxx

@@ -1338,8 +1338,8 @@ void cmGlobalGenerator::Configure()
     this->CMakeInstance->GetHomeOutputDirectory());
 
   if (this->ExtraGenerator && !this->CMakeInstance->GetIsInTryCompile()) {
-    this->CMakeInstance->IssueDiagnostic(
-      cmDiagnostics::CMD_DEPRECATED,
+    this->CMakeInstance->IssueMessage(
+      MessageType::WARNING,
       cmStrCat("Support for \"Extra Generators\" like\n  ",
                this->ExtraGenerator->GetName(),
                "\nis deprecated and will be removed from a future version "

Tests/RunCMake/ExtraGenerators/Simple-stderr.txt

@@ -1,4 +1,4 @@
-^CMake Warning \(deprecated\):
+^CMake Warning:
   Support for "Extra Generators" like
 
     [^