Deprecate repo in favor of modal-labs/modal-client

[thomasjpfan]

After this we can release a final go tag so users can see all the deprecation warnings.

Linters will catch this warning:

And in my IDE:

For us to migrate:

1. Do a release of the SDKs for modal-client.
2. Merge this PR and manually release just the GO SDK.
3. Archive this repo.

[cursor]

PR Summary

Low Risk
Primarily documentation and compiler/linter deprecation annotations; no functional runtime behavior changes beyond lint configuration.

Overview
Adds prominent migration guidance in README.md and a modal-go/v0.7.3 changelog entry announcing the move to modal-labs/modal-client and providing go get/import replacement instructions.

Marks the entire modal-go module as deprecated by adding // Deprecated: package comments across the Go sources (and go.mod), and updates modal-go/.golangci.toml to suppress specific staticcheck/ST1000 findings related to these deprecations in selected paths.

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[cursor]

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[mwaskom]

Typo? Or am I missing something?

Suggested change

	Replace all imports of `github.com/modal-labs/libmodal/modal-go` with `github.com/modal-labs/modal-client/g`.
	Replace all imports of `github.com/modal-labs/libmodal/modal-go` with `github.com/modal-labs/modal-client/go`.

[thomasjpfan]

This was a typo. 😅