Give an `Alumni` role to members who have already graduated.

[piyushchauhan]

Command: !graduation only @moderator and @admin can invoke this command

• Get the list of members whose graduation year is in the past (their role will be 🎓'20 or 🎓'19 and so on)
• Assign them Alumni role
• Mention these members in a farewell message like Farewell @Parth @Dhanush. We will remember you, your contributions, your help that you provided us with, the bakchodi we did together. Thanks a lot for everything. We hope that we keep in touch. And don't forget in taking pride of being an @Alumni (alumni role) of IIT Patna
• This command will only be run once or twice a year.

[singhayushh]

I am interested in working on this issue!