stop testing on g++-9

.github/workflows/ubuntu_install.yml

@@ -26,8 +26,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - {shared: ON}
-          - {shared: OFF}
+          shared: [ON, OFF]
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
       - name: Setup Ninja

.github/workflows/ubuntu_old.yml

@@ -21,12 +21,12 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  ubuntu-build:
+  ubuntu-build-20-04:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
         shared: [ON, OFF]
-        cxx: [g++-9, clang++-10]
+        cxx: [g++-10, clang++-10]
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
       - name: Setup Ninja

README.md

@@ -28,7 +28,7 @@ standard libraries, follow the RFC 3986. The following table illustrates possibl
 ### Requirements
 
 The project is otherwise self-contained and it has no dependency.
-A recent C++ compiler supporting C++20. We test GCC 9 or better, LLVM 10 or better and Microsoft Visual Studio 2022.
+A recent C++ compiler supporting C++20. We test GCC 12 or better, LLVM 10 or better and Microsoft Visual Studio 2022.
 
 ## Ada is fast.