ci: Be explicit about the version of linux and windows runner images

Using the -latest tag is potentially prone to errors, because an update of the tag
could break our CI. This recently happend with macos-latest, which we downgraded
to macos-12 earlier.

Using an explicit version reference makes this problem much less likely - in fact
renovate will pick up new versions once they exist and will suggest updates for it.
Thus, we will see the failures in a related PR instead of randomly everywhere.

.github/workflows/docs.yaml

@@ -25,7 +25,7 @@ concurrency:
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
     - name: Setup Nix Environment
@@ -39,7 +39,7 @@ jobs:
 
   spellcheck:
     name: Spellcheck
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
     - name: Setup Nix Environment
@@ -56,7 +56,7 @@ jobs:
   linkcheck:
     name: Linkcheck
     if: github.base_ref == 'main'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
     - name: Setup Nix Environment