ci(deps): bump checkout/upload-artifact/download-artifact majors (#1264)

Supersedes #1249, #1245 and #1247, which each rewrote the full-commit pins in
publish-snap.yaml while the same SHAs are asserted in three packaging test
files — merged separately, every one of them left those tests red.

actions/checkout v4 -> v7 (docker.yml from v6), actions/upload-artifact
v4 -> v7, actions/download-artifact v4 -> v8. New pins verified against the
upstream tag refs: checkout 3d3c42e5 = v7.0.1, upload-artifact 043fb46d =
v7.0.1, download-artifact 3e5f45b2 = v8.0.1.

download-artifact v8 changes two things on the Snap publish path, both in our
favour: a digest mismatch now fails the run instead of logging a warning, and
decompression is skipped for non-zip Content-Types (our artifact is a normal
upload-artifact zip, so unchanged). checkout v7's fork-PR block only applies to
pull_request_target/workflow_run, neither of which exists here.

.github/workflows/codeql-analysis.yml

@@ -38,7 +38,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v4
+      uses: actions/checkout@v7
 
     # Initializes the CodeQL tools for scanning. PR runs analyze the merge
     # commit checked out above (the modern default); JavaScript is