docs: update deprecation comments to reference IPIP-526

[lidel]

This PR only updates godoc, no functional changes.

It replaces references to stalled and now closed IPIP-378 with IPIP-526 which documents the historic Bitswap provider publishing API and provides a place for discussing it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.10%. Comparing base (0842ad2) to head (a79f948).
⚠️ Report is 1 commits behind head on main.

@@            Coverage Diff             @@
##             main    #1076      +/-   ##
==========================================
- Coverage   61.11%   61.10%   -0.02%     
==========================================
  Files         264      264              
  Lines       26222    26222              
==========================================
- Hits        16025    16022       -3     
- Misses       8519     8520       +1     
- Partials     1678     1680       +2     

Files with missing lines	Coverage Δ
routing/http/client/client.go	75.53% <ø> (ø)
routing/http/server/server.go	76.15% <ø> (ø)
routing/http/types/json/requests.go	52.00% <ø> (ø)
routing/http/types/json/responses.go	57.69% <ø> (ø)
routing/http/types/record_bitswap.go	47.05% <ø> (ø)

... and 5 files with indirect coverage changes

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