Change the Installer customisation to not update the selector labels

[mrsimonemms]

We recently added the ability to add custom labels, annotations and envvars to pods. For simplicity, this was added to the selector labels as well. As these are immutable, this was a mistake in retrospect.

The selectorLabels should be amended so that they only have the default labels adding, not the custom labels

This is blocking #9926 as this requires custom labels adding to the pods so that they are picked up for backup.

IMPORTANT This will introduce a potentially breaking change to the next release of Gitpod which we will need to document @lucasvaltl. The fix is as per the website. However, by doing this change, it removes this limitation for future releases.