Enable PR branch update suggestions (fixes #131)

[acul71]

Summary

• Adds newsfragment for enabling GitHub Always suggest updating pull request branches (allow_update_branch), aligning with libp2p/py-libp2p.
• Fixes Enable "Always suggest updating pull request branches" (align with py-libp2p) #131.

Maintainer action (repo setting, not in git)

After review or on merge, confirm the setting is on:

gh api repos/multiformats/py-multiaddr -X PATCH -f allow_update_branch=true
gh api repos/multiformats/py-multiaddr --jq '{allow_update_branch, default_branch}'

Test plan

• make pr
• make validate-newsfragments
• allow_update_branch is true on the repo
• (Optional) On a PR behind master with a clean merge, Update branch is visible when permitted

Made with Cursor

[acul71]

Repo setting verification

`allow_update_branch` is still false on this repository. Updating it via API requires admin on `multiformats/py-multiaddr` (push access alone is not sufficient; PATCH returns 404).

A maintainer with admin can enable it with:

```bash
gh api repos/multiformats/py-multiaddr -X PATCH -f allow_update_branch=true
gh api repos/multiformats/py-multiaddr --jq '{allow_update_branch, default_branch}'
```

Or: Settings → General → Pull Requests → Always suggest updating pull request branches.

This PR adds the newsfragment and documents the change; please flip the setting when merging (or before, so Update branch can be validated on fork PRs).

[acul71]

Hello @seetadev @sumanjeet0012 , who's got credentials to change #132 (comment) Repo setting