Add warning to RCP scenarios that are not actively maintained.

[laurinks]

Purpose of this PR

See title and this issue.

Type of change

• Minor addition to documentation

Checklist:

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

[orichters]

That might be actually something to put into https://github.com/remindmodel/remind/blob/develop/scripts/start/checkFixCfg.R, creating an actual warning that shows up in the log in case you use it.

[laurinks]

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