Revert "[CI] Add GCC 15."

This reverts commit 0bc446c.

Author: vyazelenko

.github/workflows/ci.yml

@@ -131,7 +131,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: [ '11', '12', '13', '14', '15' ]
+        version: [ '11', '12', '13', '14' ]
     env:
       CC: gcc-${{ matrix.version }}
       CXX: g++-${{ matrix.version }}