ci: pin GitHub Actions to commit SHAs (scylladb#474)

## Summary
- Pin all external GitHub Actions to full commit SHAs to reduce supply
chain attack surface
- Upgrade outdated actions to their latest versions

**This PR was generated automatically. Please verify that GitHub Actions
work as expected with these changes before merging.**

Reference: scylladb/scylladb#29421

Author: wprzytula

.github/workflows/docs-pages.yaml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with:
           ref: ${{ github.event.repository.default_branch }}
           persist-credentials: false
@@ -27,7 +27,7 @@ jobs:
         run: sudo apt-get update && sudo apt-get install -y doxygen
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v8.1.0
+        uses: astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b # v8.1.0
         with:
           enable-cache: true
           cache-dependency-glob: "docs/uv.lock"
@@ -47,7 +47,7 @@ jobs:
             .
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v4.3.3
+        uses: actions/upload-artifact@65462800fd760344b1a7b4382951275a0abb4808 # v4.3.3
         with:
           name: github-pages
           path: ${{ runner.temp }}/artifact.tar
@@ -73,4 +73,4 @@ jobs:
     steps:
       - name: Deploy to GitHub Pages
         id: deployment
-        uses: actions/deploy-pages@v4
+        uses: actions/deploy-pages@cd2ce8fcbc39b97be8ca5fce6e763baed58fa128 # v5.0.0

.github/workflows/docs-pr.yaml

@@ -19,13 +19,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
         with:
           persist-credentials: false
           fetch-depth: 0
 
       - name: Install uv
-        uses: astral-sh/setup-uv@v8.1.0
+        uses: astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b # v8.1.0
         with:
           working-directory: docs
           enable-cache: true