Updates GitHub Actions workflows for Node 20 deprecation and fixes the iOS SPM build on Flutter 3.44.

[nicolas-chaix-didomi]

• Replace actions-js/push@master with stefanzweifel/git-auto-commit-action@v6
  in update.yml — it was the only remaining action still running on
  Node 20. All other actions (checkout@v6, setup-java@v5,
  upload-artifact@v7, github-script@v9) are already on Node 24.

• Fix iOS SPM build on Flutter 3.44 by checking out into a
  didomi_sdk/ subdirectory in all iOS jobs (build.yml, test.yml,
  release.yml). Flutter 3.44 derives the SwiftPM package identity from
  the plugin's root directory name (where pubspec.yaml lives). Since
  this repo is named flutter, the generated SPM symlink was named
  Flutter instead of didomi_sdk, causing SwiftPM to refuse the
  override:

  unable to override package 'didomi_sdk' because its identity
  'flutter' doesn't match override's identity 'didomi_sdk'

  Checking out into didomi_sdk/ makes the directory name match the
  plugin name, which is what SwiftPM expects. No Package.swift change
  required.

• Tighten the Package.swift sed in update_native_sdks.sh to
  anchor on the didomi-ios-sdk-spm package line, so future SPM
  dependencies with their own from: clauses won't be accidentally
  rewritten by the version bump script.

• Gitignore Flutter SPM build artifacts (.build/, .swiftpm/,
  **/Flutter/ephemeral/, **/xcshareddata/swiftpm/) at both repo and
  example/ios/ levels.