deprecation(io): deprecate @std/io

[tomas-zijdemans]

Adds @deprecated JSDoc tags to all 21 exports of @std/io, with a migration target in each tag. Step 1 of #7258.

The package was retired in spirit when #5003 closed ("no longer stabilizing @std/io, to nudge people to use @std/streams instead"), but no export says so. Editors, deno lint, and the JSR docs surface nothing, and JSR counts 131 packages that still depend on it. After this PR, every import site gets a strikethrough and a pointer to the replacement.

The tags follow the migration table from #7258:

• APIs with a direct replacement name it: Buffer points to @std/streams, copy to ReadableStream.pipeTo, readAll to toBytes, the stream converters to the native .readable / .writable resource properties.
  The five sync exports (readAllSync, writeAllSync, iterateReaderSync, ReaderSync, WriterSync) have no web-streams equivalent. Their tags link What's happening with writeAll and writeAllSync? #6284 instead of promising a replacement that doesn't exist.
• Every tag links Proposal: deprecate @std/io #7258 for the full story.
  No runtime changes. Doc comments only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (cdb83c6) to head (1943a87).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7259      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         618      617       -1     
  Lines       51596    51593       -3     
  Branches     9340     9340              
==========================================
- Hits        49035    49031       -4     
  Misses       2021     2021              
- Partials      540      541       +1     

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