Post comment when codeflow PR cannot be updated due to conflict

[premun]

Context

In #4162 and #4163, we added a detection of when a conflict exists on the branch we're flowing the code to (e.g. the already existing PR branch).
In that case, we're throwing a new kind of exception (ConflictInPrBranchException). However, PCS does not understand this yet.

Goal

Catch this exception in the right place (somewhere in PullRequestUpdater?) and post a comment on the PR that code flow is stopped due to an update.
Potentially also pause the updates to the PR by noting down together with the PR-tracking metadata in Redis that this is the case and stop updating the PR again.
Only wait for it to get closed to stop tracking it.

Alternatively, we could try future updates again (in case the conflict disappears) and resume updating but not sure this is really needed.