http2: send 0 as the last stream ID in client GOAWAY

[cristiangirlea]

The last stream ID in a GOAWAY frame refers to streams initiated by
the peer that receives it (RFC 9113, Section 6.8). The client disables
server push, so the server never initiates streams, and the client
should send 0. sendGoAway sent the client's next stream ID instead,
which is odd, and nghttp2 rejects the frame with a PROTOCOL_ERROR.

This affects the implementation used before Go 1.27, or with the
http2legacy build tag.

Fixes golang/go#80759

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This PR (HEAD: 046500d) has been imported to Gerrit for code review.

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