CalendarView high contrast fixes - header area button styles + item background/stroke rest state

[kimsealls]

Description

• Consulted with the high contrast designer on the calendar view header and chevron buttons - these should all follow default ghost button styling in high contrast (the same style as command bar button - visible backplate on hover and press)
• Created a resource for navigation header button rest state text to support default ghost button styling
• While reviewing with the designer, we noticed a background and border showing in high contrast for all items at rest. Matched these styles to default styling and made them transparent

Motivation and Context

Buttons are now in alignment with the overall high contrast color system.

How Has This Been Tested?

Validated changes with the high contrast designer; controls test app

Screenshots (if appropriate):

https://microsoft.sharepoint-df.com/:f:/r/teams/windows/Confidential/Shared%20Documents/Pull%20Request%20Assets/CalendarHighContrast?csf=1&web=1&e=TTFBks

[ranjeshj]

/azp run

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