Update redoc to the latest version in the REST API docs

[ThorbenLindhauer]

Acceptance Criteria (Required on creation)

• redoc is updated to the latest version

Hints

• ensure that http-cache-semantics is updated to a version >= 4.1.1

Links

• https://jira.camunda.com/browse/SEC-239

Breakdown

### Pull Requests
- [ ] https://github.com/camunda/camunda-bpm-platform/pull/3686

Dev2QA handover

• [ ] Does this ticket need a QA test and the testing goals are not clear from the description? Add a Dev2QA handover comment

[tasso94]

redoc-cli is deprecated. Use @redocly/cli instead.

[tasso94]

Redoc 2.1.1 has been released: https://github.com/Redocly/redoc/releases/tag/v2.1.1

[tasso94]

• @redocly/cli enforces to load the javascript library from a CDN.
  • This is unacceptable for us since it comes with compliance problems (e.g., user data is shared with the redoc company similarly to using Google Fonts).
• The new approach uses the redoc dependency directly and creates a zero-dependency HTML file with the help of a simple NodeJS script.

Result: https://stage.docs.camunda.org/rest/camunda-bpm-platform/7.20-SNAPSHOT/