CI: bump actions/checkout and actions/upload-artifact v4 -> v7

Latest majors; clears the pending Dependabot bumps so the repo goes
public with a clean slate. setup-msys2@v2 is current.

Claude-Session: https://claude.ai/code/session_01GULf6knbsqfsnW39tMr9WT

.github/workflows/ci.yml

@@ -36,7 +36,7 @@ jobs:
   validate:
     runs-on: ubuntu-22.04
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
         with:
           persist-credentials: false
 
@@ -74,7 +74,7 @@ jobs:
       run:
         shell: msys2 {0}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
         with:
           persist-credentials: false
 
@@ -96,7 +96,7 @@ jobs:
         run: ./build-pc.sh ntsc-final
 
       - name: Upload executable
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: ge007-ntsc-final
           path: build-pc/ge007*