refactor: modernize code

[gammazero]

• apply go fix modernizers from Go 1.26
• require go1.25 or later
• go1.26 is required to run sharness tests

[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.71%. Comparing base (6aa643a) to head (3aa666e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pinning/remote/client/openapi/client.go	0.00%	9 Missing ⚠️
pinning/remote/client/openapi/api_pins.go	0.00%	5 Missing ⚠️
chunker/gen/main.go	0.00%	3 Missing ⚠️
pinning/remote/client/openapi/model_status.go	0.00%	3 Missing ⚠️
ipld/unixfs/test/utils.go	0.00%	2 Missing ⚠️
pinning/remote/client/client.go	0.00%	2 Missing ⚠️
routing/http/filters/filters.go	0.00%	2 Missing ⚠️
bitswap/network/httpnet/pinger.go	0.00%	1 Missing ⚠️
gateway/assets/test/main.go	0.00%	1 Missing ⚠️
namesys/namesys.go	0.00%	1 Missing ⚠️
... and 6 more

@@            Coverage Diff             @@
##             main    #1105      +/-   ##
==========================================
+ Coverage   61.68%   61.71%   +0.02%     
==========================================
  Files         265      265              
  Lines       26523    26513      -10     
==========================================
+ Hits        16362    16363       +1     
+ Misses       8474     8466       -8     
+ Partials     1687     1684       -3     

Files with missing lines	Coverage Δ
bitswap/client/internal/session/cidqueue.go	92.30% <100.00%> (ø)
bitswap/testinstance/testinstance.go	85.18% <100.00%> (ø)
files/multipartfile.go	81.81% <100.00%> (ø)
gateway/assets/templates.go	72.09% <100.00%> (ø)
gateway/handler.go	78.58% <100.00%> (-0.05%)	⬇️
gateway/handler_defaults.go	56.42% <100.00%> (ø)
gateway/headers.go	100.00% <100.00%> (ø)
gateway/serve_http_content.go	48.10% <100.00%> (ø)
ipld/merkledag/coding.go	78.84% <100.00%> (ø)
ipld/merkledag/merkledag.go	74.22% <100.00%> (-0.52%)	⬇️
... and 21 more

... and 7 files with indirect coverage changes

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