fix: Ignore pushes of update steps for read only sessions

[juliushaertl]

My suggestion for now would be to block any update steps for read only sessions according to what is described in the y.js protocol docs.

https://github.com/yjs/y-protocols/blob/master/PROTOCOL.md#handling-read-only-users

Ideally we'd of course also prevent sending those in the frontend code, but for now this seems the most reasonable quick fix.

[max-nextcloud]

Did not test - but code looks good.

[juliushaertl]

/backport to stable29

[juliushaertl]

/backport to stable28

[juliushaertl]

/backport to stable27

[backportbot]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b backport/5895/stable27

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 672d3a8f

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/5895/stable27

Error: No changes found in backport branch

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Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.