update: remove RFC3896 `conn_type` warning

[michotross257]

Remove warning about underscores in conn_type.

When secrets backend is initially loaded using a URI a Connection object is created using the URI (e.g. Vault) part of which involves normalizing conn_type by replacing dashes with underscores (see Connection._normalize_conn_type). Then when the connection is saved to secret cache (see Connection.get_connection_from_secrets) the connection with a normalized conn_type is transformed back into a URI which causes the RFC3896 warning related to underscores appearing in conn_type.

For example, a google-cloud-platform connection in Vault defined as a URI (where conn_type is google-cloud-platform) will generate this warning about underscores in the conn_type when referenced by an Airflow component.

{
    "conn_uri": "google-cloud-platform:..."
}

Another option could be to move this warning to Connection._normalize_conn_type.

---

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