http: reject HTTP major version switch mid connection

A connection that has seen an HTTP major version now refuses any other
major HTTP version in future responses. Previously, a HTTP/1.

http: reject HTTP major version switch mid connection

A connection that has seen an HTTP major version now refuses any other
major HTTP version in future responses. Previously, a HTTP/1.x
connection would just silently accept HTTP/2 or HTTP/3 in the status
lines as long as it had support for those built-in. It would then just
lead to confusion and badness.

Indirectly Spotted by CodeSonar which identified a duplicate assignment
in this function.

Add test 471 to verify

Closes #13421

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