Backport of fix race condition in string generator helper into release/1.13.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #19875 to be assessed for backporting due to the inclusion of the label backport/1.13.x.

The below text is copied from the body of the original PR.

---

A data race in vault/helper/random.(*StringGenerator).validateConfig() was exposed by new text coverage that was recently introduced.

WARNING: DATA RACE
Read at 0x00c005d77000 by goroutine 10158:
  github.com/hashicorp/vault/helper/random.(*StringGenerator).validateConfig()
      /home/circleci/go/src/github.com/hashicorp/vault/helper/random/string_generator.go:243 +0x3e6

and

Previous write at 0x000006f28ae0 by goroutine 10155:
  github.com/hashicorp/vault/helper/random.(*StringGenerator).validateConfig()
      /home/circleci/go/src/github.com/hashicorp/vault/helper/random/string_generator.go:238 +0x293

Failure: https://app.circleci.com/pipelines/github/hashicorp/vault/51983/workflows/d654323c-baa8-4985-9838-a4131f8900f5/jobs/607405/steps

---

Overview of commits

• 88cbf25