chore: add deprecation notice for --link

[bpmct]

code-server --link has not been updated for a while, and some users have reported connection issues with it (see #2734)

While we will continue to host the --link servers, this PR adds a warning that we are not officially maintaining this version of --link. In the future, we will likely release an improved version, using a new architecture and codebase. However, there is no timeline for this.

[github-actions]

✨ Coder.com for PR #4562 deployed! It will be updated on every commit.

• Host: https://codercom-4ys7ngq9r-codercom.vercel.app/docs/code-server
• Last deploy status: success
• Commit: 9ad76d7
• Workflow status: https://github.com/cdr/code-server/actions/runs/1517387758

[codecov]

Codecov Report

Merging #4562 (9ad76d7) into main (3d46604) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #4562   +/-   ##
=======================================
  Coverage   65.44%   65.44%           
=======================================
  Files          30       30           
  Lines        1664     1664           
  Branches      335      335           
=======================================
  Hits         1089     1089           
  Misses        487      487           
  Partials       88       88           

Impacted Files	Coverage Δ
src/node/cli.ts	80.62% <0.00%> (ø)

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[jsjoeio]

Thanks for taking this on @bpmct! 🙌