Use ubuntu-latest for checks not relying on any specific version

For the checks running simple shell scripts, any Ubuntu version should
work, so switch them to use the latest one to avoid having to update
them every time when an existing version is dropped by GitHub Actions.

.github/workflows/code_checks.yml

@@ -14,7 +14,7 @@ permissions:
 
 jobs:
   check-unix:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Check Spelling
 
     steps:
@@ -31,7 +31,7 @@ jobs:
 
 
   check-whitespace:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Check Whitespace
 
     steps:
@@ -69,7 +69,7 @@ jobs:
 
 
   check-mixed-eol:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Check Mixed EOL
 
     steps:
@@ -85,7 +85,7 @@ jobs:
             ./misc/scripts/check_mixed_eol.sh
 
   check-cxx-style:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Check C++ Style
 
     steps:
@@ -103,7 +103,7 @@ jobs:
             fi
 
   check-allheaders:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     name: Check All Headers In allheaders.h
 
     steps: