Prevent installing/upgrade for a namespace when we detect an installation in another namespace

[NickLarsenNZ]

From #379

We might want to handle the case of:

• install release 24.11 with operator_namespace = "ns1"
• install/upgrade release 25.3 with operator_namespace = "ns2"

Because:

• The CRDs are cluster scopes, so they would be overwritten, but...
• The operators in the old NS will remain.

I'm fine with us only supporting upgrading in the same namespace (moving namespaces should probably be an uninstall and install). So I guess we'd just need to search for operators in any namespace (labels might be the safest way for detecting).