fix: Upgrade GitHub Actions for Node 24 compatibility

Merge #3932

## Summary

Upgrade GitHub Actions to their latest versions to ensure compatibility with Node 24, as Node 20 will reach end-of-life in April 2026.

## Changes

| Action | Old Version(s) | New Version | Release | Files |
|--------|---------------|-------------|---------|-------|
| `actions/checkout` | [`v5`](https://github.com/actions/checkout/releases/tag/v5) | [`v6`](https://github.com/actions/checkout/releases/tag/v6) | [Release](https://github.com/actions/checkout/releases/tag/v6) | analyze-releases-for-adk-docs-updates.yml, check-file-contents.yml, discussion_answering.yml, isort.yml, pr-triage.yml, pyink.yml, python-unit-tests.yml, stale-bot.yml, triage.yml, upload-adk-docs-to-vertex-ai-search.yml |

## Context

Per [GitHub's announcement](https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/), Node 20 is being deprecated and runners will begin using Node 24 by default starting March 4th, 2026.

### Why this matters

- **Node 20 EOL**: April 2026
- **Node 24 default**: March 4th, 2026
- **Action**: Update to latest action versions that support Node 24

### Security Note

Actions that were previously pinned to commit SHAs remain pinned to SHAs (updated to the latest release SHA) to maintain the security benefits of immutable references.

### Testing

These changes only affect CI/CD workflow configurations and should not impact application functionality. The workflows should be tested by running them on a branch before merging.

Co-authored-by: Xiang (Sean) Zhou <[email protected]>
COPYBARA_INTEGRATE_REVIEW=#3932 from salmanmkc:upgrade-github-actions-node24 0ffbb0b
PiperOrigin-RevId: 845813166

Author: 2 people

.github/workflows/analyze-releases-for-adk-docs-updates.yml

@@ -16,7 +16,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/check-file-contents.yml

@@ -24,7 +24,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 2
 

.github/workflows/discussion_answering.yml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/isort.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 2
 

.github/workflows/pr-triage.yml

@@ -20,7 +20,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/pyink.yml

@@ -26,7 +26,7 @@ jobs:
 
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 2
 

.github/workflows/python-unit-tests.yml

@@ -29,7 +29,7 @@ jobs:
 
     steps:
     - name: Checkout code
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
 
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v6

.github/workflows/stale-bot.yml

@@ -18,7 +18,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/triage.yml

@@ -24,7 +24,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Set up Python
         uses: actions/setup-python@v6

.github/workflows/upload-adk-docs-to-vertex-ai-search.yml

@@ -13,7 +13,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
 
       - name: Clone adk-docs repository
         run: git clone https://github.com/google/adk-docs.git /tmp/adk-docs