QDoc: Add 20.1 as supported Clang version

LLVM 20.1.0 is scheduled for release on 11 March. QDoc has been tested
when linked against a release candidate. It compiles, its tests pass,
and it generates the documentation for Qt without regressing. Thus, we
can be reasonably certain that QDoc works as expected when linked
against Clang libraries from LLVM 20.

To allow Qt's build system to consider the CMake files that LLVM 20
ships, add the correct version string to `.cmake.conf`.

Pick-to: 6.9.0 6.9 6.8.3 6.8
Task-number: QTBUG-134304
Change-Id: I7b875fffce0066eb141e8e51e3800b2c86eac331
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

Author: paulwicking

.cmake.conf

@@ -3,7 +3,7 @@ set(QT_REPO_MODULE_PRERELEASE_VERSION_SEGMENT "alpha1")
 set(QT_EXTRA_INTERNAL_TARGET_DEFINES "QT_NO_AS_CONST=1")
 set(QDOC_MINIMUM_CLANG_VERSION "17")
 set(QDOC_SUPPORTED_CLANG_VERSIONS
-    "19.1" "18.1" "17.0"
+    "20.1" "19.1" "18.1" "17.0"
 )
 list(APPEND QT_EXTRA_INTERNAL_TARGET_DEFINES "QT_NO_FOREACH=1")
 list(APPEND QT_EXTRA_INTERNAL_TARGET_DEFINES "QT_NO_CONTEXTLESS_CONNECT=1")