Drop old compiler versions from the build matrix.

As per GitHub Actions, `ubuntu-latest` now uses Ubuntu Jammy and
LLVM/Clang versions 13 through 15 are what's currently available
in the nightly packages on the LLVM site; 16 is in development.

According to the GNU site, GCC versions 10 through 12 are what's
currently supported; 13 is in development.

Change-Id: Ia6c5ec12b0248e7df0a37d397748d11dc1748d3b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60830
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e7bc5f0..807a5ca 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -27,7 +27,7 @@
     strategy:
       fail-fast: false
       matrix:
-        ver: [9, 10, 11, 12, 13, 14, 15]
+        ver: [13, 14, 15]
     env:
       CC: clang-${{ matrix.ver }}
       CXX: clang++-${{ matrix.ver }}
@@ -47,7 +47,7 @@
     strategy:
       fail-fast: false
       matrix:
-        ver: [6, 7, 8, 9, 10, 11, 12]
+        ver: [10, 11, 12]
     env:
       CC: gcc
       CXX: g++