RFR: 7081: Update Copyright year to 2021

[Guru Hb]

Updated Copyright year to 2021 for below 
1. License show in "About" dialog
2. Copyright year in License file 
3. Update site landing page 
4. added a helper script which can be used to update copyright year for modified git files.

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Commit messages:
 - 7081: Update Copyright year to 2021

Changes: https://git.openjdk.java.net/jmc/pull/190/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=190&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7081
  Stats: 881 lines in 313 files changed: 24 ins; 0 del; 857 mod
  Patch: https://git.openjdk.java.net/jmc/pull/190.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/190/head:pull/190

PR: https://git.openjdk.java.net/jmc/pull/190