[nrf fromtree] ci: doc: Use Ubuntu 20.04 runner image

This commit updates the documentation build and publish workflows to
use a specific runner image version, ubuntu-20.04, instead of the
latest version in order to prevent any potential breakages due to the
'latest' version change by GitHub.

Signed-off-by: Stephanos Ioannidis <[email protected]>
(cherry picked from commit 18064425236cee67c6fb68617702c5862b945844)
Signed-off-by: Thomas Stilwell <[email protected]>

Author: stephanosio

.github/workflows/doc-build.yml

@@ -35,8 +35,9 @@ env:
 jobs:
   doc-build-html:
     name: "Documentation Build (HTML)"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 45
+
     concurrency:
       group: doc-build-html-${{ github.ref }}
       cancel-in-progress: true
@@ -121,7 +122,7 @@ jobs:
 
   doc-build-pdf:
     name: "Documentation Build (PDF)"
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     container: texlive/texlive:latest
     timeout-minutes: 45
     concurrency:

.github/workflows/doc-publish-pr.yml

@@ -13,7 +13,7 @@ on:
 jobs:
   doc-publish:
     name: Publish Documentation
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     if: |
       github.event.workflow_run.event == 'pull_request' &&
       github.event.workflow_run.conclusion == 'success' &&

.github/workflows/doc-publish.yml

@@ -16,7 +16,7 @@ on:
 jobs:
   doc-publish:
     name: Publish Documentation
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     if: |
       github.event.workflow_run.event != 'pull_request' &&
       github.event.workflow_run.conclusion == 'success' &&