(#21264) Update rgen dependency to 0.6.5

Commit ec6b51a8ded9245df9606a57d53674c62bddc11e updated the rgen dependency in
the Gemfile. This commit makes the same dependency explicit in the various
packages (deb, rpm, gem). We pin the version hard at 0.6.5 to prevent potential
breakages with future versions of rgen.