Add deprecation warning for `-m` option

[ann0see]

Adds a deprecation warning if the server is started with -m.

Short description of changes

CHANGELOG: The -m/--htmlstatus option is considered deprecated and has been replaced by JSON RPC's jamulusserver/getClients method. The -m option will be removed in future.

Context: Fixes an issue?

Related to: https://github.com/jamulussoftware/jamulus/issues/3362

Does this change need documentation? What needs to be documented and how?

Yes. Part of https://github.com/jamulussoftware/jamulus/issues/3362

Status of this Pull Request

What is missing until this pull request can be merged?

Approval.

Checklist

• [x] I've verified that this Pull Request follows the general code principles
• [x] I tested my code and it does what I want
• [x] My code follows the style guide
• [ ] I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• [ ] I've filled all the content above