Use a specific version of Ubuntu in workflows

Using ubuntu-latest might result in incompatibilities when this image
changes to a different version of Ubuntu. For example, the Ubuntu 24.04
image no longer includes imagemagick, meaning that we'll have to install
it manually when using Ubuntu 24.04.

.github/workflows/auto-author-assign.yml

@@ -11,6 +11,6 @@ jobs:
   assign-author:
     name: Auto-assign pull request author
     if: github.repository_owner == 'consuldemocracy'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - uses: toshimaru/auto-author-assign@v2.1.1

.github/workflows/db_schema.yml

@@ -12,7 +12,7 @@ permissions:
 
 jobs:
   schema:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     services:
       postgres:
         image: postgres:13.16

.github/workflows/docker.yml

@@ -6,7 +6,7 @@ permissions:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout code
         uses: actions/checkout@v4

.github/workflows/linters.yml

@@ -6,7 +6,7 @@ permissions:
 
 jobs:
   linters:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:

.github/workflows/pronto.yml

@@ -3,7 +3,7 @@ on: [pull_request]
 
 jobs:
   pronto:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     if: github.event.pull_request.head.repo.full_name == github.event.pull_request.base.repo.full_name && github.actor != 'dependabot[bot]'
     steps:
       - name: Checkout code

.github/workflows/tests.yml

@@ -15,7 +15,7 @@ env:
 
 jobs:
   tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     timeout-minutes: 60
     services:
       postgres:
@@ -76,7 +76,7 @@ jobs:
   coveralls:
     permissions:
       contents: none
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
     needs: tests
     steps:
       - name: Finish coveralls