Pin the doctest workflow to Ubuntu 22.04 (pythonGH-125236)

(cherry picked from commit 7a10cde)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: AA-Turner

.github/workflows/reusable-docs.yml

@@ -95,7 +95,7 @@ jobs:
   # Run "doctest" on HEAD as new syntax doesn't exist in the latest stable release
   doctest:
     name: 'Doctest'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     timeout-minutes: 60
     steps:
     - uses: actions/checkout@v4