Update Github actions checkout and upload-artifact to v6 for Node 24 compatibility

Summary:
Upgrade GitHub Actions to their latest versions to ensure compatibility with Node 24, as Node 20 EOL is April 2026.

| Action | Old Version(s) | New Version |
|--------|---------------|-------------|
| `actions/checkout` | [`v4`](https://github.com/actions/checkout/releases/tag/v4) | [`v6`](https://github.com/actions/checkout/releases/tag/v6) |
| `actions/upload-artifact` | [`v4`](https://github.com/actions/upload-artifact/releases/tag/v4) | [`v6`](https://github.com/actions/upload-artifact/releases/tag/v6) |

[GitHub announced](https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/) that Node 20 is being deprecated and runners will begin using Node 24 by default starting 2026-03-04.

**Node 20 EOL**: April 2026
**Node 24 becomes default on Github Actions**: 2026-03-04

Reviewed By: bigfootjon

Differential Revision: D89332222

fbshipit-source-id: b8793f6a7a2aaac6f93cd8419e9afa49c5489bee

Author: hchokshi

build/fbcode_builder/getdeps.py

@@ -1174,7 +1174,7 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
                 # && is not supported on default windows powershell, so use cmd
                 out.write("      shell: cmd\n")
 
-            out.write("    - uses: actions/checkout@v4\n")
+            out.write("    - uses: actions/checkout@v6\n")
 
             build_type_arg = ""
             if override_build_type:
@@ -1365,7 +1365,7 @@ def write_job_for_platform(self, platform, args):  # noqa: C901
                 f"--final-install-prefix /usr/local\n"
             )
 
-            out.write("    - uses: actions/upload-artifact@v4\n")
+            out.write("    - uses: actions/upload-artifact@v6\n")
             out.write("      with:\n")
             out.write("        name: %s\n" % manifest.name)
             out.write("        path: _artifacts\n")