refactor: update go version to 1.26 and apply modernizers

[gammazero]

• Update minimum go version to 1.26
• Apply modernizers with go fix ./...

[codecov]

Codecov Report

❌ Patch coverage is 65.71429% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.31%. Comparing base (25b1db8) to head (afbcdb6).

Files with missing lines	Patch %	Lines
pinning/remote/client/openapi/utils.go	0.00%	8 Missing ⚠️
blockstore/blockstore.go	0.00%	3 Missing ⚠️
namesys/dns_resolver.go	0.00%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main    #1211      +/-   ##
==========================================
+ Coverage   64.18%   70.31%   +6.12%     
==========================================
  Files         269      269              
  Lines       27193    27186       -7     
==========================================
+ Hits        17455    19117    +1662     
+ Misses       8024     8022       -2     
+ Partials     1714       47    -1667     

Files with missing lines	Coverage Δ
gateway/errors.go	85.82% <100.00%> (+2.88%)	⬆️
ipld/unixfs/unixfs.go	88.47% <100.00%> (+5.94%)	⬆️
ipns/record.go	85.16% <100.00%> (+9.79%)	⬆️
path/resolver/resolver.go	84.55% <100.00%> (+6.50%)	⬆️
retrieval/state.go	97.07% <100.00%> (+1.14%)	⬆️
routing/http/client/measures.go	78.37% <100.00%> (+8.37%)	⬆️
namesys/dns_resolver.go	79.78% <0.00%> (+6.10%)	⬆️
blockstore/blockstore.go	63.12% <0.00%> (+7.09%)	⬆️
pinning/remote/client/openapi/utils.go	0.00% <0.00%> (ø)

... and 173 files with indirect coverage changes

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