fix(NcDateTimePicker): warn about usage of non-standard formatting tokens

[odzhychko]

Their usage was never intentionally supported.
The warning is added to warn users about their future removal.

☑️ Resolves

Prepares resolving #8931.

Completely resolving #8931 might involve throwing an error if such tokens are enabled.

This PR only adds a warning.

🖼️ Screenshots

No visual change

🚧 Tasks

• The issue this PR is resolving was suggested by me, so before merging someone should check whether [NcDateTimePicker]: forbid using not standard formatting tokens #8931 even makes sense.

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.44%. Comparing base (e6a0ba1) to head (b01c15c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8934      +/-   ##
==========================================
+ Coverage   52.18%   52.44%   +0.26%     
==========================================
  Files         120      121       +1     
  Lines        4350     4372      +22     
  Branches     1298     1302       +4     
==========================================
+ Hits         2270     2293      +23     
+ Misses       1765     1764       -1     
  Partials      315      315              

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