Rename default branch references from master to main

[spoenemann]

Summary

Follow-up to the rename of this repository's default branch from master to main, configured in eclipse-sprotty/.eclipsefdn#6. GitHub's rename redirects old URLs and retargets open pull requests, but does not rewrite workflow files, so this PR updates everything in the repository that names the branch:

• .github/workflows/build.yml and publish.yml trigger on main instead of master.
• publish.yml filters maintenance branches with maintenance/* instead of maintenance-*, matching the publish environment's branch policy in the organisation configuration and the convention used across the Sprotty projects.
• ADR-0005 documents the new trigger branches.
• blob/master links in the README and CHANGELOG files point directly to main (the old links only kept working through the redirect).

Links into other repositories (sprotty-theia, sprotty-vscode, sprotty-server, eclipse/.github) are unchanged because those repositories keep their default branch.

Merge order

1. Merge Rename default branch of sprotty to main .eclipsefdn#6. Otterdog renames master to main, and this PR is retargeted automatically.
2. Merge this PR right afterwards. Until then, pushes to main trigger neither the build nor the publish workflow.

Updating local clones

After the rename, run this in every clone that still tracks master:

git branch -m master main
git fetch --prune origin
git branch -u origin/main main
git remote set-head origin -a

If you work from a fork, rename the branch there too (repository settings → Branches, or gh api -X POST repos/<user>/sprotty/branches/master/rename -f new_name=main) and then run git fetch --prune <fork-remote>.

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