Remove golang.org/x/exp dependency and upgrade to Go 1.21

[stv0g]

Fixes #77

[codecov]

Codecov Report

Merging #80 (76c9114) into master (885c82a) will increase coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   62.24%   62.60%   +0.36%     
==========================================
  Files          23       23              
  Lines        1666     1666              
==========================================
+ Hits         1037     1043       +6     
+ Misses        483      479       -4     
+ Partials      146      144       -2     

Files Changed	Coverage Δ
config/arguments.go	54.46% <ø> (ø)
config/handler_cmd.go	0.00% <ø> (ø)
config/handler_keyout.go	0.00% <ø> (ø)
config/handler_wg_wgctrl.go	0.00% <ø> (ø)
conn_udp.go	66.66% <ø> (ø)
handshake_initiator.go	73.72% <ø> (ø)
internal/test/server_standalone.go	62.71% <ø> (ø)
peer.go	92.68% <ø> (ø)
server.go	72.10% <ø> (+2.57%)	⬆️

[stv0g]

Disabling lint workflow for now due to golangci/golangci-lint#3922 (comment)