ci: bump checkout and setup-node to v6 for Node 24

GitHub deprecated actions running on Node.js 20. The release workflow
emitted a warning because actions/checkout@v4 and actions/setup-node@v4
still run on Node.js 20, which is forced to Node.js 24 by default from
June 16th, 2026 and removed from runners on September 16th, 2026.

Bump both to v6, since v5 onward already runs on Node.js 24, which
clears the deprecation warning. oven-sh/setup-bun@v2 is already the
latest major and was not flagged, so it stays unchanged.

Signed-off-by: Kevin Cui <bh@bugs.cc>

Author: BlackHole1

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
   check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - uses: oven-sh/setup-bun@v2
         with:
           bun-version-file: .bun-version

.github/workflows/release.yml

@@ -43,7 +43,7 @@ jobs:
       VERSION_BUMP: ${{ inputs.version_bump }}
       GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           fetch-depth: 0 # need full history + tags to compute the next version
           fetch-tags: true
@@ -52,7 +52,7 @@ jobs:
         with:
           bun-version-file: .bun-version
 
-      - uses: actions/setup-node@v4
+      - uses: actions/setup-node@v6
         with:
           # OIDC trusted publishing needs Node >= 22.14.0; "22" resolves to the latest 22.x.
           node-version: "22"