Add warning to RCP scenarios that are not actively maintained.

[laurinks]

Purpose of this PR

See title and this issue.

Type of change

• [x] Minor addition to documentation

Checklist:

• [x] My code follows the coding etiquette
• [x] I performed a self-review of my own code
• [x] I explained my changes within the PR, particularly in hard-to-understand areas
• [x] I checked that the in-code documentation is up-to-date
• [ ] I adjusted the reporting in remind2 where it was needed
• [x] I adjusted forbiddenColumnNames in readCheckScenarioConfig.R in case the PR leads to deprecated switches
• [x] All automated model tests pass (FAIL 0 in the output of make test)
• [x] The changelog CHANGELOG.md has been updated correctly

[laurinks]

Again what learned! ;) Thanks for the suggestion, @orichters ! I added it.