.github/workflows: test newer clang

The current images do not have Clang 16 anymore; use the current 3 latest.

Change-Id: Ie86c24aa139fd02325f02f3350ed921231758332
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63591
Reviewed-by: Alan Donovan <adonovan@google.com>

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4945638..52cf49b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -34,7 +34,7 @@
     strategy:
       fail-fast: false
       matrix:
-        ver: [16, 17, 18]
+        ver: [18, 19, 20]
     env:
       CC: clang-${{ matrix.ver }}
       CXX: clang++-${{ matrix.ver }}