<Swing Dev> RFR: 8251377: [macos11] JTabbedPane selected tab text is barely legible

[Prasanta Sadhukhan]

On macOS 11 (bigsur) beta, using the Swing Aqua Look and Feel, the text of the selected JTabbedPane tab title text is just a light gray outline of white text on a white background. The macOS 11 design inverted from dark background / light text to light background / dark text, 
Proposed fix is to use dark color in light background so I used lightGray instead of "white" which is being used now, if we are on BigSur.

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Commit messages:
 - 8251377: [macos11] JTabbedPane selected tab text is barely legible

Changes: https://git.openjdk.java.net/jdk/pull/1182/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1182&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251377
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1182.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1182/head:pull/1182

PR: https://git.openjdk.java.net/jdk/pull/1182