Update go.mod and github test to go 1.21

[zhiyipanuber]

This updates go.mod files to use Go 1.21.

It also updates the CI to use 1.21 to test.

[codecov]

Codecov Report

Patch coverage: 76.71% and project coverage change: -0.24% ⚠️

Comparison is base (0c11cc2) 88.78% compared to head (6c64580) 88.54%.
Report is 4 commits behind head on dev.

❗ Current head 6c64580 differs from pull request most recent head d35f2ed. Consider uploading reports for the commit d35f2ed to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #899      +/-   ##
==========================================
- Coverage   88.78%   88.54%   -0.24%     
==========================================
  Files          43       43              
  Lines        4440     4505      +65     
==========================================
+ Hits         3942     3989      +47     
- Misses        379      394      +15     
- Partials      119      122       +3     

Files Changed	Coverage Δ
outbound.go	87.28% <33.33%> (+0.14%)	⬆️
messages.go	91.58% <50.00%> (-6.27%)	⬇️
mex.go	72.88% <76.00%> (+0.36%)	⬆️
connection.go	85.49% <88.88%> (+0.50%)	⬆️
inbound.go	81.50% <100.00%> (-0.89%)	⬇️
relay.go	85.77% <100.00%> (+0.06%)	⬆️
relay_messages.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[rabbbit]

this should be 1.20 - see https://github.com/uber-go/fx/pull/1111/files

[AllenLuUber]

ohh yea. Good callout. @zhiyipanuber can you patch this?

[AllenLuUber]

actually talked to @rabbbit and @zhiyipanuber offline, this should be fine for now as this future code won't build with version lower than 1.20.0.