rephrase text in banner on Legacy RFCs

[alicerusso]

Description

Current: This RFC was published on the Legacy stream.
Suggested: This RFC is labeled as "Legacy"; it was published before a formal source was recorded.

Example: https://datatracker.ietf.org/doc/rfc872/
Scope: This banner appears on 2083 RFCs.
Rationale: Current text sounds odd because "Legacy" is the label used when there is not source information; an RFC cannot be "published on the Legacy stream". "Legacy" is defined as 'The RFC was published before a formal source (e.g., working group or stream) existed or was recorded.' on https://www.rfc-editor.org/source-definitions/.

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