Switch github runners to use ubuntu-latest

Github actions has deprecated Ubuntu 20.04 runners:

actions/runner-images#11101

Switch to just running on the latest ubuntu image for our runners to
avoid future maintenance burden and ensure compatibility with up to date
toolchains.

Signed-off-by: Daniel Burgener <[email protected]>

Author: dburgener

.github/workflows/checkers.yml

@@ -10,7 +10,7 @@ on: ["push", "pull_request", "workflow_dispatch"]
 
 jobs:
   codespell:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     - name: Checkout from github

.github/workflows/epub.yml

@@ -10,7 +10,7 @@ on: ["push", "pull_request", "workflow_dispatch"]
 
 jobs:
   epub:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     - name: Checkout from GitHub

.github/workflows/html.yml

@@ -10,7 +10,7 @@ on: ["push", "pull_request", "workflow_dispatch"]
 
 jobs:
   html:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     - name: Checkout from GitHub

.github/workflows/pdf.yml

@@ -10,7 +10,7 @@ on: ["push", "pull_request", "workflow_dispatch"]
 
 jobs:
   pdf:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     - name: Checkout from GitHub