Mark repository as unmaintained/archive it

[rklec]

In https://github.com/microsoft/RulesEngine/issues/609#issuecomment-2186958404 and https://github.com/microsoft/RulesEngine/issues/604 I just found out this repo is not maintained anymore. @abbasc52 confirmed this in https://github.com/microsoft/RulesEngine/issues/604#issuecomment-2181168220

Could you please mark it as unmaintained aka archive it? See https://docs.github.com/en/repositories/archiving-a-github-repository/archiving-repositories

Also adding a link/alternative to a supported project would obviously be good.

[YogeshPraj]

@rklec please let me know if you want me to merge any changes to main branch.