RFR: 8237504: Update copyright header for files modified in 2020

[Kevin Rushforth]

Update last modified year of copyright headers for files modified in 2020.

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Commit messages:
 - 8237504: Update copyright header for files modified in 2020

Changes: https://git.openjdk.java.net/jfx/pull/203/files
 Webrev: https://webrevs.openjdk.java.net/jfx/203/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8237504
  Stats: 144 lines in 144 files changed: 0 ins; 0 del; 144 mod
  Patch: https://git.openjdk.java.net/jfx/pull/203.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/203/head:pull/203

PR: https://git.openjdk.java.net/jfx/pull/203