Use math/rand/v2 in place of math/rand

Describe the enhancement:

In Go 1.22 we got a new package math/rand/v2 that replaces the aging math/rand.

Some improvements include better performance and scalability as well as safer algorithms. See Evolving the Go Standard Library with math/rand/v2.

Since our go.mod file requires go1.22.4 minimum we can safely depend on this new package.

What is the definition of done?

All occurrences of math/rand are replaced with math/rand/v2.

Grepping for math/rand usage I got:

$ rg -l '"math/rand"' | xargs rg 'rand\.'
main.go
39:     rand.Seed(time.Now().UnixNano())

internal/pkg/scheduler/scheduler.go
133:    return time.Duration(rand.Int63n(t))

magefile.go
2109:           bkpName := fmt.Sprintf("./env.sh-%d", rand.Int())

internal/pkg/remote/client.go
267:    rand.Shuffle(len(clients), func(i, j int) {

internal/pkg/core/backoff/equal_jitter.go
33:             nextRand: time.Duration(rand.Int63n(int64(init))), //nolint:gosec
54:     b.nextRand = time.Duration(rand.Int63n(int64(b.duration)))

testing/integration/install_test.go
337:    rand.Seed(time.Now().UnixNano())
342:            runes[i] = letters[rand.Intn(len(letters))]

internal/pkg/agent/cmd/enroll_cmd.go
707:    t := time.NewTimer(time.Duration(rand.Int63n(int64(d))))

internal/pkg/agent/application/upgrade/marker_access_test.go
162:            rune := chars[rand.Intn(len(chars))]

internal/pkg/agent/application/upgrade/artifact/download/http/verifier_test.go
114:    first := rand.Intn(len(tt))
115:    second := rand.Intn(len(tt))

rand.Seed(time.Now().UnixNano()) rand.Seed is deprecated since 1.20 and removed in v2, see https://github.com/elastic/elastic-agent/pull/5334
rand.Int stays in v2, no changes needed.
rand.Intn stays in v2 as rand.IntN
rand.Int63n(x) can now be written as rand.N(x) as stated in the blog post.
rand.Shuffle stays the same and gains some perf improvements as stated in the blog post.

elastic / elastic-agent

Use math/rand/v2 in place of math/rand #5335