Remove deprecation diagnostic supppression for dtostrf

[jboynes]

Have the tests use an implementation of dtostrf which checks for the largest buffer needed by the String class.
Document the risks of that implementaton and add a TODO not to rely on this function in the production code.

As this no longer uses the deprecated sample implementation, the warning suppression can be removed as it is not compiled.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.03% ⚠️

Comparison is base (66aa7db) 95.52% compared to head (fdda28c) 95.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
- Coverage   95.52%   95.50%   -0.03%     
==========================================
  Files          16       15       -1     
  Lines        1072     1067       -5     
==========================================
- Hits         1024     1019       -5     
  Misses         48       48              

see 1 file with indirect coverage changes

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