feat(buttons): update button colors to match current Teams theme

[notandrew]

before:

after:

adding darker shadow to purple button:
before:

after:

dark theme hover state:
before:

after:

high contrast default buttons:
before:

after:

[codecov]

Codecov Report

Merging #1933 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1933      +/-   ##
==========================================
- Coverage   70.27%   70.26%   -0.01%     
==========================================
  Files         893      893              
  Lines        7892     7893       +1     
  Branches     2309     2310       +1     
==========================================
  Hits         5546     5546              
- Misses       2333     2334       +1     
  Partials       13       13

Impacted Files	Coverage Δ
...es/teams-dark/components/Button/buttonVariables.ts	0% <ø> (ø)	⬆️
...high-contrast/components/Button/buttonVariables.ts	0% <ø> (ø)	⬆️
...src/themes/teams/components/Button/buttonStyles.ts	2.85% <0%> (-0.09%)	⬇️

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[layershifter]

LGTM, @codepretty can you please check?