maintenance:install --> remove oracle option: database-connection-string

[mmattel]

The database-connection-string is only used for Oracle which is no longer supported.
It should be either removed or "inactivated" to avoid any harm.
The docs for oc11 (wip) do not show Oracle anymore, but with maintenance:install --help, the option is still visible.

Note that also the --database option currently includes the oci value for Oracle which should be removed from the value list.

grep -rn database-connection-string

@phil-davis @DeepDiver1975