Immutable github releases

JRleaser 1.21.0 now has support immutable github releases.

GitHub Immutable releases: https://github.blog/changelog/2025-10-28-immutable-releases-are-now-generally-available/
JReleaser 1.21.0 release notes: https://github.com/jreleaser/jreleaser/releases/tag/v1.21.0

The current flake lockfile is for rev `2fb006b87f04c4d3bdf08cfdbc7fab9c13d94a15`, updated in #6511. This uses version 1.20.0 of jreleaser: https://github.com/NixOS/nixpkgs/blob/2fb006b87f04c4d3bdf08cfdbc7fab9c13d94a15/pkgs/by-name/jr/jreleaser-cli/package.nix.

As soon as this updated to 1.21.0, immutable releases can be enabled. This required two actions:

* Enable immutable releases on the github repository. This is done under `Settings > General > Releases > Enable immutable releases`.
* Modify the jreleaser github release config to support immutable releases:
```diff
release:
github:
owner: INRIA
+++ immutableRelease: true
prerelease:
pattern: .*beta.*
changelog:
formatted: ALWAYS
preset: conventional-commits
format: '- {{commitShortHash}} {{commitTitle}}'
contributors:
format: '- {{contributorName}} ({{contributorUsernameAsLink}})'
hide:
contributors:
- '[bot]'
- 'GitHub'
```

This modification makes JReleaser use the workflow `create draft > upload assets > release release` instead of the default `create release (not draft) > upload assets`. As, when the immutable release has been created, it cannot be modified and no assets can be uploaded.

Start with the flake lockfile and the JReleaser GitHub release configuration described in the issue. Check the JReleaser version, update it to 1.21.0, and review the release workflow configuration. Done means the repository setting and JReleaser configuration both support immutable releases and assets are uploaded before publication.

Written by the indexing model from the issue text.