Run most workflows on ubuntu-24.04 (#7875)

Github is currently rolling out ubuntu-latest as ubuntu-24.04. Manage
that switch explicitly by running most jobs on 24.04

https://github.com/actions/runner-images/issues/10636

This keeps the release on 20.04 to ensure released binaries can run on
older operating systems (because of CGO/glibc versions)

.github/workflows/boulder-ci.yml

@@ -27,7 +27,7 @@ jobs:
   #  tags and 5 tests there would be 10 jobs run.
   b:
     # The type of runner that the job will run on
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     strategy:
       # When set to true, GitHub cancels all in-progress jobs if any matrix job fails. Default: true
@@ -95,7 +95,7 @@ jobs:
         run: ${{ matrix.tests }}
 
   govulncheck:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     strategy:
       fail-fast: false
 
@@ -117,7 +117,7 @@ jobs:
         run: go run golang.org/x/vuln/cmd/govulncheck@latest ./...
 
   vendorcheck:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
     strategy:
       # When set to true, GitHub cancels all in-progress jobs if any matrix job fails. Default: true
       fail-fast: false
@@ -153,7 +153,7 @@ jobs:
     permissions:
       contents: none
     if: ${{ always() }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     name: Boulder CI Test Matrix
     needs:
       - b