http2: improve error when server sends HTTP/1

[colega]

If for some reason the server sends an HTTP/1.1 response
starting with "HTTP/1.1 " (9 bytes), http2 transport interprets that as
a valid frame header for an expected payload of ~4MB, and fails with
non-descriptive messages like "unexpected EOF".

This could happen, for example, if ALPN is misconfigured on the server's
load balancer.

This change attempts to improve that feedback by noting in the error
messages whenever the frame header was exactly the "HTTP/1.1 " bytes.

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Patch Set 1:

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Patch Set 4: Code-Review+2

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Patch Set 4: Commit-Queue+1

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Patch Set 4:

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Patch Set 4:

This CL has failed the run. Reason:

Tryjob golang/try/x_net-go1.23-linux-386 has failed with summary (view all results):

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• go on release-branch.go1.23 (commit 45aade7)
• net on master (change 623155)

Build or test failure, click here for results.

To reproduce, try gomote repro 8720920604437015537.

Additional links for debugging:

• Output for test golang.org/x/net module

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Patch Set 4: LUCI-TryBot-Result-1

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Patch Set 4:

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Patch Set 5: Commit-Queue+1

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Patch Set 5:

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Patch Set 5: -Commit-Queue

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Patch Set 5:

This CL has passed the run

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Patch Set 5: LUCI-TryBot-Result+1

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Patch Set 5: Auto-Submit+1 Code-Review+1

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Patch Set 5: Code-Review+2

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This PR is being closed because golang.org/cl/623155 has been merged.