Update License Management's release process doc, mention Python 2 jobs

Problem to solve

In license-management!56, the pipeline of license-management has significantly changed in order to build Docker images where Python 2 is the default. The release process documentation does not reflect these changes.

Also, license-management!55 introduced dotenv files that set the default values of other environment variables, like FEATURE_RUBY_REPORT and LM_REPORT_VERSION.

Proposal

Update the release process documentation to explain how version 2 jobs build Docker images where Python 2 is the default.

Alternatively, add this to the project README.

Who can address the issue

Any backend developer working on the devopssecure stage.

Other links/references

https://gitlab.com/gitlab-org/gitlab-ee/issues/12032

/cc @xlgmokha @gonzoyumo