[AppBar] Mark MDCAppBarTextColorAccessibilityMutator as deprecated.

[jverkoey]

Part of https://github.com/material-components/material-components-ios/issues/4806

[material-automation]

API diff detected the following changes

AppBar

MDCAppBarTextColorAccessibilityMutator

removed class: MDCAppBarTextColorAccessibilityMutator

removed method: -mutate: in MDCAppBarTextColorAccessibilityMutator

[jverkoey]

API diff tool is incorrectly picking up the deprecated APIs as being removed. Will investigate separately.

[codecov-io]

Codecov Report

Merging #4807 into develop will decrease coverage by 0.34%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4807      +/-   ##
===========================================
- Coverage    53.93%   53.58%   -0.35%     
===========================================
  Files          324      324              
  Lines        28076    28074       -2     
===========================================
- Hits         15143    15044      -99     
- Misses       12933    13030      +97

Impacted Files	Coverage Δ
components/AppBar/src/MDCAppBarViewController.h	33.33% <ø> (ø)	⬆️
components/AppBar/src/MDCAppBarViewController.m	56.81% <ø> (ø)	⬆️
...exibleHeader/src/MDCFlexibleHeaderViewController.m	64.41% <0%> (-8.58%)	⬇️
...ponents/ButtonBar/src/private/MDCButtonBarButton.m	93.33% <0%> (-6.67%)	⬇️
...vigation/src/private/MDCBottomNavigationItemView.m	69.96% <0%> (-4.95%)	⬇️
...ents/BottomNavigation/src/MDCBottomNavigationBar.m	62.59% <0%> (-4.16%)	⬇️
components/TextFields/src/MDCTextField.m	74.11% <0%> (-3.32%)	⬇️
components/NavigationBar/src/MDCNavigationBar.h	70% <0%> (-2.73%)	⬇️
components/ButtonBar/src/MDCButtonBar.m	64.49% <0%> (-1.82%)	⬇️
components/Tabs/src/private/MDCItemBar.m	42.75% <0%> (-0.52%)	⬇️
... and 8 more

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