Update WiredTiger source code to include 2021 copyright notices

As a WT developer, I want to update WiredTiger source code to include 2021 copyright notices.

Note from last year's ticket:

See dist/s_copyright for a script that checks copyright notices. You'll still need to manually review files, but that script gets you most of the way there.

This ticket requires the following work:

• Create and run a script to update all files with copyright notices from 2020 to 2021. You can tweak the existing dist/s_copyright script to do this.
• Manually review the files to ensure they are updated as expected.
• Update this ticket with the script you used for future reference!