ci: pin actions to the node20 generation for the forge runner

The forge's forgejo-runner (v6.3.1) only supports node runtimes up to node20,
but resolves bare `uses:` from code.forgejo.org where checkout@v6 (and the
other @v5/v6/v7 actions) declare `runs.using: node24` -> "must be one of
[... node20 ...], got node24" -> job failed before any step ran.

- checkout@v6->v4, cache@v5->v4, docker/{setup-buildx@v4->v3, login@v4->v3,
  build-push@v7->v6}: all node20.
- jdx/mise-action isn't mirrored on code.forgejo.org; reference it by full
  github URL, pinned to @v2 (node20).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/deploy.yml

@@ -24,13 +24,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v4
 
       - name: Install mise
-        uses: jdx/mise-action@v4
+        uses: https://github.com/jdx/mise-action@v2
 
       - name: Cache Cargo artifacts
-        uses: actions/cache@v5
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry/index/
@@ -63,13 +63,13 @@ jobs:
     needs: [check]
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v4
 
       - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v4
+        uses: docker/setup-buildx-action@v3
 
       - name: Login to the forge container registry
-        uses: docker/login-action@v4
+        uses: docker/login-action@v3
         with:
           # Forgejo auto-provides GITHUB_TOKEN to the job; it can push packages
           # for this repo's owner (uberbau). No external registry / PAT needed.
@@ -78,7 +78,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push container
-        uses: docker/build-push-action@v7
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: true

.github/workflows/push.yml

@@ -13,13 +13,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v4
 
       - name: Install mise
-        uses: jdx/mise-action@v4
+        uses: https://github.com/jdx/mise-action@v2
 
       - name: Cache Cargo artifacts
-        uses: actions/cache@v5
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry/index/
@@ -51,13 +51,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v4
 
       - name: Install mise
-        uses: jdx/mise-action@v4
+        uses: https://github.com/jdx/mise-action@v2
 
       - name: Cache Cargo artifacts
-        uses: actions/cache@v5
+        uses: actions/cache@v4
         with:
           path: |
             ~/.cargo/registry/index/