Pin ubuntu runner images to 20.04 version

Instead of running GitHub Actions on ubuntu-latest (which recently
changed to 22.04) pin a specific version of 20.04.

When I was first making the workflows I used ubuntu-latest as a way
to have motivation to have the workflow code modern. With more
experience I see that it unnecessarily adds work maintaining the repo.

Author: avelanarius

.github/workflows/docs-pages.yaml

@@ -16,7 +16,7 @@ on:
 
 jobs:
   release:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v3

.github/workflows/docs-pr.yaml

@@ -15,7 +15,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v3

.github/workflows/tests-reports-4x@v1.yml

@@ -10,7 +10,7 @@ on:
       - completed
 jobs:
   report:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 5
 
     steps:

.github/workflows/tests-reports@v1.yml

@@ -10,7 +10,7 @@ on:
       - completed
 jobs:
   report:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 5
 
     steps:

.github/workflows/tests@v1.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
 
     strategy:
@@ -33,7 +33,7 @@ jobs:
 
   verify:
     name: Full verify
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
 
     strategy:
@@ -56,7 +56,7 @@ jobs:
 
   unit-tests:
     name: Unit tests
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
 
     steps:
@@ -88,7 +88,7 @@ jobs:
 
   setup-integration-tests:
     name: Setup ITs
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     timeout-minutes: 2
 
     steps:
@@ -113,7 +113,7 @@ jobs:
 
   cassandra-integration-tests:
     name: Cassandra ITs
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: [setup-integration-tests]
     timeout-minutes: 90
 
@@ -170,7 +170,7 @@ jobs:
 
   scylla-integration-tests:
     name: Scylla ITs
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     needs: [setup-integration-tests]
     timeout-minutes: 90