Deprecate pre-LLVM 14 build configurations in GH actions

After the full switch onto LLVM 14, retaining build compability with earlier
versions has become unfeasible. Resort to LLVM 14-15 as the only verified
versions.

Author: Artem Gindinson

.github/workflows/build-IGC.yml

@@ -50,15 +50,6 @@ jobs:
         ubuntu_version: [ '20.04', '22.04' ]
         llvm_ver: [ 14 ]
         include:
-          - ubuntu_version: '20.04'
-            llvm_ver: 11
-            experimental: true
-          - ubuntu_version: '22.04'
-            llvm_ver: 12
-            experimental: true
-          - ubuntu_version: '22.04'
-            llvm_ver: 13
-            experimental: true
           - ubuntu_version: '22.04'
             llvm_ver: 15
             experimental: true
@@ -110,18 +101,6 @@ jobs:
         llvm_ver: [ 14 ]
         compiler: [ gcc, clang ]
         include:
-          - ubuntu_version: '20.04'
-            llvm_ver: 11
-            compiler: gcc
-            experimental: true
-          - ubuntu_version: '22.04'
-            llvm_ver: 12
-            compiler: gcc
-            experimental: true
-          - ubuntu_version: '22.04'
-            llvm_ver: 13
-            compiler: gcc
-            experimental: true
           - ubuntu_version: '22.04'
             llvm_ver: 15
             compiler: gcc