This PR prepares another minor release that includes the changes from this week.

Breaking changes:

• optimade.server.mappers.ResourceMapper has been renamed as optimade.server.mappers.BaseResourceMapper (#213).

New features:

• Validation of all mandatory filter examples present in the specification (#213, @ml-evs.)
• New invoke task to scrape the filter examples from the specification (and new submodule optimade.validator.data for loading them (#213, @ml-evs, with bug fixes from @CasperWA in #225).
• Dependabot is now used to ensure requirements are updated in a timely manner and new versions of django (v3) and elasticsearch-dsl (v7) are now supported (@CasperWA, #218, #219).
• The GitHub action for the validator has been moved out of this repository, and over to Materials-Consortia/optimade-validator-action (#224, @CasperWA).

Bug fixes:

• Fixed handling of nested queries involving AND/OR and NOT for the mongo backend (#221, @ml-evs).