Add CMake option to disable account migration

[Rello]

In branded clients, it can be required not to migrate existing (e.g. private) accounts when the client is setup the first time.

• add DISABLE_ACCOUNT_MIGRATION CMake option
• compile out legacy account migration code and CLI flag when disabled

https://github.com/nextcloud-gmbh/brander/issues/997

[Rello]

/backport to stable-3.18

[backportbot]

The backport to stable-3.18 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable-3.18
git pull origin stable-3.18

# Create the new backport branch
git checkout -b backport/8633/stable-3.18

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 31c4c260

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/8633/stable-3.18

Error: Failed to clone repository: Failed to create working tree: Preparing worktree (new branch 'backport/8633/stable-3.18')
fatal: not a valid object name: 'origin/stable-3.18'

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

[Rello]

/backport to stable-3.17

[Rello]

/backport to stable-3.16