CI: Add LLVM flang 21.1.0

Author: bonachea

.github/workflows/build.yml

@@ -42,6 +42,12 @@ jobs:
             network: smp
             # https://hub.docker.com/r/snowstep/llvm/tags
             container: snowstep/llvm:noble
+          - os: ubuntu-24.04
+            compiler: flang
+            version: 21
+            network: smp
+            # https://hub.docker.com/r/phhargrove/llvm-flang/tags
+            container: phhargrove/llvm-flang:21.1.0-4
           - os: ubuntu-24.04
             compiler: flang
             version: 20
@@ -78,6 +84,11 @@ jobs:
             version: 20
             network: udp
             container: phhargrove/llvm-flang:20.1.0-1
+          - os: ubuntu-24.04
+            compiler: flang
+            version: 21
+            network: udp
+            container: phhargrove/llvm-flang:21.1.0-4
 
     container:
       image: ${{ matrix.container }}