dns/dnsmessage: update SVCB packing to prohibit name compression

[fortuna]

This commit updates the dns/dnsmessage package to correctly handle SVCB record
packing by prohibiting name compression for the Target field.

It turns out the compression of names in RDATA is prohibited (see
https://datatracker.ietf.org/doc/html/rfc3597#section-4), except for well-known record
types (those defined in https://datatracker.ietf.org/doc/html/rfc1035).

The SVCB RFC actually explicitly calls that out
(https://datatracker.ietf.org/doc/html/rfc9460#section-2.2), so it was an oversight
in my part in https://go.dev/cl/710736.

Updates golang/go#43790

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Message from Gopher Robot:

Patch Set 1:

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Message from Vinicius Fortuna:

Patch Set 1:

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Patch Set 1:

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Message from Vinicius Fortuna:

Patch Set 1:

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Patch Set 2:

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Message from Vinicius Fortuna:

Patch Set 2:

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Message from Vinicius Fortuna:

Patch Set 2: Code-Review+1

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Message from t hepudds:

Patch Set 2: Commit-Queue+1

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Patch Set 2:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-10-22T21:22:45Z","revision":"20de74c4a68f29f2c67b1f63c3f7cf790a3b4838"}

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Patch Set 2: -Commit-Queue

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Patch Set 2: Code-Review+2

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Patch Set 2: Auto-Submit+1 Code-Review+1

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This PR is being closed because golang.org/cl/713900 has been merged.