Context:
7.8.0 reached end of life in December 2021: https://www.elastic.co/support/eol
and 7.9.0 will in February 2022

AT:

• Elasticsearch 7.8.0/7.9.0 is removed from the supported versions in the version checker ESVersionChecker
• Supported Environment in docs are updated accordingly
• removed supported versions are added to the announcements
• the 7.8.0/7.9.0 stage is removed from the elasticsearch compatibility job
• removed 7.8.0/7.9.0 support is listed in the migration notes