WebUI provides deprecated source kinds in Add Source dialog

[pashagolub]

patroni-continous-discovery and patroni-namespace-discovery were deprecated in v4.

Currently supported sources kinds are:

const (
	SourcePostgres           Kind = "postgres"
	SourcePostgresContinuous Kind = "postgres-continuous-discovery"
	SourcePgBouncer          Kind = "pgbouncer"
	SourcePgPool             Kind = "pgpool"
	SourcePatroni            Kind = "patroni"
)