request: run C8 REST API synchronization process for any version

[pepopowitz]

Currently the C8 REST API sync process only runs against the camunda/camunda main branch, to update the next version of the docs.

#4956 will add confusion to this, since we will be managing two sets of next docs. When that lands, the sync process will run for version 8.8, but not 8.7.

To avoid mistakes and confusion, we could alter the sync-rest-api-docs workflow to accept an input parameter, specifying the version to synchronize.

Implementation notes

• see https://damienaicheh.github.io/github/actions/2022/01/20/set-dynamic-parameters-github-workflows-en.html for a walkthrough of using walkthrough inputs
• We would need a specific branch/tag to pull for each version. We already have a home for each version of the spec in the docs, and the ability to pass the version number into the generation script.

[pepopowitz]

See #4979 for a concern re: sidebars files in non-next versions. There will be some faffing we need to do to copy the generated sidebars file back up to the top-level sidebars JSON file, replacing only a subtree of it. Blech.