build(gradle): update build.gradle

[ferdicus]

Remove obsolete supportLibVersion - no longer required with AndroidX

Summary

Removing the supportLibVersion variable that was used in app/build.gradle to suffix version numbers on android support libraries.
As react-native moved to androidX with the release of 0.60 this is no longer necessary.

Changelog

[Android] [Removed] - Remove supportLibVersion variable in build.gradle

Test Plan

No tests were run locally

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@osdnk has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[osdnk]

Looks good. Hope it won't break internals

[react-native-bot]

This pull request was successfully merged by @ferdicus in fee7f06.

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