Cirrus-CI: use llvm16 job by default and make llvm15 manual

With commit e048f78b21 the default tool chain is Clang/LLVM 16, so
follow along in the llvm pkg used by Cirrus-CI.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation

.cirrus.yml

@@ -16,14 +16,14 @@ env:
 task:
   matrix:
   - name: amd64-llvm15 World and kernel build and boot smoke test
+    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
+    trigger_type: manual
     env:
       TARGET: amd64
       TARGET_ARCH: amd64
       TOOLCHAIN: llvm15
       TOOLCHAIN_PKG: ${TOOLCHAIN}-lite
   - name: amd64-llvm16 World and kernel build and boot smoke test
-    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
-    trigger_type: manual
     env:
       TARGET: amd64
       TARGET_ARCH: amd64