Encode Python 3.12 requirement in PEP-723 block (#3445)

## Why

I tried running `gh_report.py` with system Python and it failed with:
```
AttributeError: 'PosixPath' object has no attribute 'walk'
```

This was added in Python 3.12.

## Tests

I can now run the script through `uv run`.

Author: pietern

tools/gh_parse.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# ///
 """
 Analyze downloaded GH logs and print a report. Use gh_report.py instead of this script directly.
 """

tools/gh_report.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# ///
 """
 Download integration logs artifacts for a given run id (--run RUNID) or commit (--commit) and call gh_parse.py on those to print the report.
 

tools/lintdiff.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# ///
 """
 Drop in replacement for golangci-lint that runs it only on changed packages.
 

tools/start_integration_tests.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# ///
 """
 Start integration test jobs for PRs by non-team members that are approved by team members.
 """

tools/update_github_links.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# ///
 """Update PR references in changelog files.
 
 1. Convert occurrences of `#1234` to the canonical markdown link

tools/validate_whitespace.py

@@ -1,4 +1,7 @@
 #!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# ///
 import os
 import re
 import sys