multiple: switch to math/rand/v2

[marcoferrer]

Fixes #7293

This PR replaces usage of math/rand with math/rand/v2.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.76%. Comparing base (5f178a8) to head (cc929ad).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7711      +/-   ##
==========================================
- Coverage   81.77%   81.76%   -0.02%     
==========================================
  Files         361      361              
  Lines       27826    27825       -1     
==========================================
- Hits        22756    22752       -4     
- Misses       3865     3867       +2     
- Partials     1205     1206       +1     

Files with missing lines	Coverage Δ
balancer/endpointsharding/endpointsharding.go	66.92% <100.00%> (ø)
balancer/grpclb/grpclb_picker.go	97.43% <100.00%> (ø)
balancer/leastrequest/leastrequest.go	88.88% <ø> (ø)
balancer/pickfirst/pickfirst.go	84.42% <ø> (+0.68%)	⬆️
balancer/rls/internal/adaptive/adaptive.go	100.00% <ø> (ø)
balancer/roundrobin/roundrobin.go	100.00% <100.00%> (ø)
balancer/weightedroundrobin/balancer.go	80.58% <ø> (ø)
internal/backoff/backoff.go	87.09% <ø> (ø)
internal/resolver/dns/dns_resolver.go	90.86% <100.00%> (ø)
internal/transport/http2_server.go	90.93% <100.00%> (-0.82%)	⬇️
... and 8 more

... and 17 files with indirect coverage changes

[easwars]

This is going to be a little painful. But could you please rename this import as rand "math/rand/v2", so that we can track the package name from use sites. Thank you.