CI: stockdotnet lanes to use 22.04, not 24.04

Ubuntu24.04 doesn't have dotnet6 packages anymore, but dotnet8.
As we don't want the stable branch to be .NET8 compatible, we
just don't test this scenario.

Author: knocte

.github/workflows/CI.yml

@@ -302,10 +302,10 @@ jobs:
     - name: integration tests
       run: make update-servers
 
-  linux24-vanilla--stockdotnet6-only:
-    runs-on: ubuntu-24.04
+  linux22-vanilla--stockdotnet6-only:
+    runs-on: ubuntu-22.04
     container:
-      image: "ubuntu:24.04"
+      image: "ubuntu:22.04"
     steps:
     - uses: actions/checkout@v1
     # can't use the option below because of error "Input 'submodules' not supported when falling back to download using the GitHub REST API. To create a local Git repository instead, add Git 2.18 or higher to the PATH."
@@ -344,10 +344,10 @@ jobs:
 
         find . -type f -name "*.fsx" | xargs -t -I {} dotnet fsxc {}
 
-  linux24-vanilla--stockdotnet6-and-newmono:
-    runs-on: ubuntu-24.04
+  linux22-vanilla--stockdotnet6-and-newmono:
+    runs-on: ubuntu-22.04
     container:
-      image: "ubuntu:24.04"
+      image: "ubuntu:22.04"
     steps:
     - uses: actions/checkout@v1
     # can't use the option below because of error "Input 'submodules' not supported when falling back to download using the GitHub REST API. To create a local Git repository instead, add Git 2.18 or higher to the PATH."
@@ -388,10 +388,10 @@ jobs:
 
         find . -type f -name "*.fsx" | xargs -t -I {} dotnet fsxc {}
 
-  linux24-vanilla--stockdotnet6-and-stockmono:
-    runs-on: ubuntu-24.04
+  linux22-vanilla--stockdotnet6-and-stockmono:
+    runs-on: ubuntu-22.04
     container:
-      image: "ubuntu:24.04"
+      image: "ubuntu:22.04"
     steps:
     - uses: actions/checkout@v1
     # can't use the option below because of error "Input 'submodules' not supported when falling back to download using the GitHub REST API. To create a local Git repository instead, add Git 2.18 or higher to the PATH."
@@ -602,13 +602,13 @@ jobs:
     - linux24-github--dotnet-and-newmono
     - linux24-vanilla--stockmono-only
     - linux24-vanilla--newmono-only
-    - linux24-vanilla--stockdotnet6-only
-    - linux24-vanilla--stockdotnet6-and-stockmono
-    - linux24-vanilla--stockdotnet6-and-newmono
     - linux22-github--dotnet-and-mono
     - linux22-github--dotnet-and-newmono
     - linux22-vanilla--stockmono-only
     - linux22-vanilla--newmono-only
+    - linux22-vanilla--stockdotnet6-only
+    - linux22-vanilla--stockdotnet6-and-stockmono
+    - linux22-vanilla--stockdotnet6-and-newmono
     - windows--dotnet6-and-legacyFramework
     - windows--legacyFramework-only
     - windows--dotnet6-only