Update clang runners

Author: mborland

.github/workflows/ci.yml

@@ -246,7 +246,7 @@ jobs:
             linkflags: -stdlib=libc++
           - toolset: clang
             compiler: clang++-16
-            cxxstd: "03,11,14,17,20,2b"
+            cxxstd: "03,11,14,17,20"
             os: ubuntu-latest
             container: ubuntu:22.04
             install:
@@ -286,6 +286,26 @@ jobs:
               - "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-19 main"
             source_keys:
               - "https://apt.llvm.org/llvm-snapshot.gpg.key"
+          - toolset: clang
+            compiler: clang++-20
+            cxxstd: "03,11,14,17,20,2b"
+            os: ubuntu-24.04
+            install:
+              - clang-20
+            sources:
+              - "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-20 main"
+            source_keys:
+              - "https://apt.llvm.org/llvm-snapshot.gpg.key"
+          - toolset: clang
+            compiler: clang++-21
+            cxxstd: "03,11,14,17,20,2b"
+            os: ubuntu-24.04
+            install:
+              - clang-21
+            sources:
+              - "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-21 main"
+            source_keys:
+              - "https://apt.llvm.org/llvm-snapshot.gpg.key"
 
           - toolset: clang
             cxxstd: "03,11,14,17,20,2b"