Add a prefix to deploy tokens

Prefixes Group & Project Deploy Tokens with `gldt-` following the
guidance at
https://docs.gitlab.com/ee/development/secure_coding_guidelines.html#token-prefixes.

GitLab applies a prefix to some of its generated secrets. For example, a
Personal Access Token begins with `glpat-`. This MR adds a prefix to
Deploy Tokens. It also updates our frontend secret detection which
helps prevent users from leaking tokens via Issue / MR comments.

It applies to both Group & Project tokens: `GroupDeployToken` `has_one`
`DeployToken`, and the latter is where the plaintext is generated. This
change means both types of deploy keys will be prefixed
`gldt-`.

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/376752

Changelog: changed