v1.2.1 - add strncasecmp_P(); update README.md doc

• 1.2.1 (2022-01-10)
  • Add strncasecmp_P() to pgmspace.h. See
    PR#52.
  • Add Bugs and Limitations section in
    README.md
  • Add comment in
    github/workflows/aunit_tests.yml
    that a pull_request event may be useful. Upgrade GitHub docker image to
    Ubuntu 20.04.