ci: move to actions/checkout v7 and actions/cache v6, pin ubuntu-26.04

The v4 pair targets Node 20, which GitHub now forces onto Node 24 (Node 20 is
deprecated). The Forgejo runner is v13, which runs Node 24, so both copies move
together. Pin the GitHub runner to ubuntu-26.04: ubuntu-latest migrates to it
over Oct 19 - Nov 19, 2026, and Nix supplies the toolchain either way.

.github/workflows/ci.yml

@@ -19,25 +19,25 @@ defaults:
 
 jobs:
   ci:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     timeout-minutes: 90
     permissions:
       contents: read
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v7
       - uses: cachix/install-nix-action@13d8dd58da0234aa297dedd986986ccb8e7f3e24 # v31.11.1
       - uses: nix-community/cache-nix-action@7df957e333c1e5da7721f60227dbba6d06080569 # v7.0.2
         with:
           primary-key: nix-v2-${{ hashFiles('flake.lock', '**/*.nix') }}
           restore-prefixes-first-match: nix-v2-
           gc-max-store-size-linux: 6G
-      - uses: actions/cache@v4
+      - uses: actions/cache@v6
         with:
           path: |
             ~/.gradle/caches
           key: gradle-${{ hashFiles('**/*.gradle*', '**/gradle.properties', 'gradle/libs.versions.toml', 'gradle/wrapper/gradle-wrapper.properties') }}
           restore-keys: gradle-
-      - uses: actions/cache@v4
+      - uses: actions/cache@v6
         with:
           path: .integration/music
           key: integration-music-v1