Add text size for placeholder text that matches what we do for the sky-form-control class

[Blackbaud-TrevorBurch]

No description provided.

[blackbaud-ado]

Preview build at https://developer.blackbaud.com/skyux-forms?_sp.spa=pr-input-box-placeholder-modern-size

[codecov]

Codecov Report

Merging #263 (d8dd129) into master (d346095) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #263   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines         1060      1060           
  Branches       202       202           
=========================================
  Hits          1060      1060           

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[blackbaud-ado]

Preview build at https://developer.blackbaud.com/skyux-forms?_sp.spa=pr-input-box-placeholder-modern-size