Push version to v3

[ChrisSchinnerl]

With #22 being a breaking change we should add the v3 suffix before tagging v3.0.0.

Depends on #27

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Pull request overview

This PR migrates the mux module to v2 by adding the /v2 suffix to the module path, which is required for semantic versioning in Go when releasing a breaking change (as mentioned in the referenced PR #22).

Key Changes:

• Updated module declaration to use the v2 path
• Updated internal import to reference the v3 protocol implementation under the new v2 module path

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
go.mod	Updates the module declaration from go.sia.tech/mux to go.sia.tech/mux/v2 to support semantic versioning for the upcoming v2.0.0 release
mux.go	Updates the import path for the v3 protocol implementation to go.sia.tech/mux/v2/v3 to reflect the new v2 module structure

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[lukechampine]

definitely one of the import paths of all time

maybe we should lift v3 into the top-level, and push the version to v3?

[n8mgr]

That sounds like the best idea.

[ChrisSchinnerl]

Lifted it and also added a little bonus in my latest commit to make the handshake more efficient.

[lukechampine]

should we go ahead and drop covert streams in v3?