Consider switching service chaining domain from `spin.internal` to `spin.alt`

[lann]

Looking at https://www.rfc-editor.org/rfc/rfc9476.html I think .alt might be more appropriate:

• .alt "should not be resolved using the DNS protocol"
• .internal is to be used for internal networks with DNS

(RFC9476 is only a proposal but very far along and seems almost certain to be accepted)

[itowlson]

For 3.0 we could adopt spin.alt as the documented domain but continue to accept spin.internal for backward compatibility.