refactor(internal/bytesconv): replace rand usage with crypto/rand and rand.New

[1911860538]

This commit refactors random usage to follow current best practices in Go 1.20+:

• Replaced usage of math/rand.Read, which is deprecated, with crypto/rand.Read.
  As per the deprecation notice, crypto/rand.Read is more appropriate for almost all use cases.

• Replaced rand.NewSource(time.Now().UnixNano()) with rand.New(rand.NewSource(time.Now().UnixNano())).
  rand.NewSource returns a bare Source which lacks convenience methods like Intn, Float64, etc.
  Wrapping it with rand.New provides a complete and idiomatic *rand.Rand object.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.92%. Comparing base (3dc1cd6) to head (1a45951).
Report is 131 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4259      +/-   ##
==========================================
- Coverage   99.21%   98.92%   -0.30%     
==========================================
  Files          42       44       +2     
  Lines        3182     3434     +252     
==========================================
+ Hits         3157     3397     +240     
- Misses         17       26       +9     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.85% <ø> (?)
-tags go_json	98.85% <ø> (?)
-tags nomsgpack	98.90% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.23	98.92% <ø> (?)
go-1.24	98.92% <ø> (?)
macos-latest	98.92% <ø> (-0.30%)	⬇️
ubuntu-latest	98.92% <ø> (-0.30%)	⬇️

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