Upgrade GitHub Actions: checkout v3→v6, setup-java v3→v5, cache v3→v5

Node20 is EOL April 2026; runners switch to Node24 by default June 2, 2026.

As announced in this blog post:
https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Action users must upgrade to latest versions before Node24 switchover
to avoid CI errors.

See the warnings on recent CI runs, such as
https://github.com/emichael/dslabs/actions/runs/23388642671/job/68039631981 (under
"Annotations").

We use the following actions. The new versions run on Node24 and
require Actions Runner v2.327.1+.

| Action            | Before | After | Node runtime |
|-------------------|--------|-------|--------------|
| actions/checkout  | v3     | v6    | Node24       |
| actions/setup-java| v3     | v5    | Node24       |
| actions/cache     | v3     | v5    | Node24       |

READMEs:
- https://github.com/actions/checkout/blob/main/README.md
- https://github.com/actions/setup-java/blob/main/README.md
- https://github.com/actions/cache/blob/main/README.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

.github/workflows/github-pages.yml

@@ -16,16 +16,16 @@ jobs:
     runs-on: ubuntu-latest
     concurrency: github-pages
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
-      - uses: actions/cache@v3
+      - uses: actions/cache@v5
         with:
           path: ./www/vendor/bundle
           key: ${{ runner.os }}-gems-${{ hashFiles('./www/Gemfile.lock') }}
           restore-keys: |
             ${{ runner.os }}-gems-
 
-      - uses: actions/setup-java@v3
+      - uses: actions/setup-java@v5
         with:
           distribution: 'adopt'
           java-version: '21'

.github/workflows/test-publish.yml

@@ -14,9 +14,9 @@ jobs:
       matrix:
         java: [ '21', '25' ]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-java@v3
+      - uses: actions/setup-java@v5
         with:
           distribution: 'adopt'
           java-version: ${{ matrix.java }}
@@ -30,9 +30,9 @@ jobs:
     name: Check formatting
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-java@v3
+      - uses: actions/setup-java@v5
         with:
           distribution: 'adopt'
           java-version: '21'
@@ -47,9 +47,9 @@ jobs:
     runs-on: ubuntu-latest
     concurrency: publish-handout
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v6
 
-      - uses: actions/setup-java@v3
+      - uses: actions/setup-java@v5
         with:
           distribution: 'adopt'
           java-version: '21'