Mark pkg.strings for deprecation

[isegura-eos-eng]

Preflight Checklist

• I could not find a solution in the existing issues, docs, nor discussions
• I have joined the ZITADEL chat

Describe your problem

Since go1.21 there exist a stdlib slices package. This includes the functionality that this project pkg/strings, which is the strings.Contains function. The homonymous slices function works also for strings.

Describe your ideal solution

I propose to mark this function for deprecation and remove any reference from the project code. This way package clients can still use this for v3.

Version

3.33.1

Environment

Self-hosted

Additional Context

I'm gonna submit the PR, we already discussed this briefly in: #660 (comment)

[hifabienne]

I think we can close this issue as the pr is already mergedd