Cirrus-CI: use llvm15 toolchain packages

As of commit 50d7464c3f we use llvm15 as the system toolchain, and
commit eca005d853 added compiler options incompatible with earlier
versions.  Switch to llvm15 packages.

Sponsored by:	The FreeBSD Foundation

.cirrus.yml

@@ -16,13 +16,13 @@ task:
     env:
       TARGET: amd64
       TARGET_ARCH: amd64
-      TOOLCHAIN_PKG: llvm14
+      TOOLCHAIN_PKG: llvm15
   - name: World and kernel arm64 build and boot smoke test
     trigger_type: manual
     env:
       TARGET: arm64
       TARGET_ARCH: aarch64
-      TOOLCHAIN_PKG: llvm14
+      TOOLCHAIN_PKG: llvm15
   - name: World and kernel gcc12 amd64 build and boot smoke test
     trigger_type: manual
     env: