chore: remove bonita 7.8 version

[benjaminParisel]

Since bonita 2021.1 is out-of-support, we remove the latest version from site and generate static documentation for 7.8

Theses PR should be merged before this documentation-site PR.

• [x] 2021.1 no more propagated: #535 merge the PR prior merging the others
• [x] Documentation content: https://github.com/bonitasoft/bonita-doc/pull/2342
• [x] Rest-documentation-site update: https://github.com/bonitasoft/bonita-rest-documentation-site/pull/19
• [x] Archived Bonita 7.8 https://github.com/bonitasoft/bonita-doc/pull/2343

:heavy_check_mark: Default branch on bonita-doc is now 2021.2 :heavy_check_mark: Algolia config is updated to remove 7.8

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