License dialog: Always use date handler

[lukasbestle]

This PR …

Fixes

• Fix renewal date formatting in the license dialog when using a custom date handler Panel: Kirby license date format wonky (4.0.2) #6134

Ready?

• Unit tests for fixed bug/feature
• In-code documentation (wherever needed)
• Tests and checks all pass

For review team

• Add changes to release notes draft in Notion
• Add to website docs release checklist (if needed) 

[distantnative]

Wondering whether we really want to add it as parameter to each method or rather define that the License methods will use the date handler (so passing it inside the methods to Str::date.

[lukasbestle]

For our use, yes. But devs might want to hook into this for custom dashboards etc. Allowing a $handler argument is more consistent with the rest of the system.

[bastianallgeier]

I think it only feels a bit like unnecessary public features first but on the other hand I agree about the consistency.