Add Clang 18 to the build matrix.

... and remove Clang 15 so that we continue to
test with only the three most recent releases.

Change-Id: Ib1eaa20b2c988bb53027dd3e01551d5d95058e24
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62830
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 445da78..a7107db 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -34,7 +34,7 @@
     strategy:
       fail-fast: false
       matrix:
-        ver: [15, 16, 17]
+        ver: [16, 17, 18]
     env:
       CC: clang-${{ matrix.ver }}
       CXX: clang++-${{ matrix.ver }}