[puppet] Propose retiring old stable branches(Stein, Rocky and Queens)

Hello, In puppet repos we have plenty of stable branches still open, and the oldest is now stable/queens. However recently we haven't seen many backports proposed to stein, rocky and queens [1], so I'll propose retiring these three old branches now. Please let me know if anybody is interested in keeping any of these three. Note that currently CI jobs are broken in these three branches and it is likely we need to investigate the required additional pinning of dependent packages. If somebody is still interested in maintaining these old branches then these jobs should be fixed. [1] https://review.opendev.org/q/(project:%255Eopenstack/puppet-.*)AND(NOT+proje...)) Thank you, Takashi Kajinami