Improve auto removal email notification

Quoting from https://lists.debian.org/debian-devel/2022/05/msg00231.html

"In general, it's massively spamming me, just it should be spamming you as well (well, anyone maintaining a large amount of packages).

I recieved 536 mails (2 days ago)...

On 5/27/22 09:48, Paul Gevers wrote: Patches welcome. Code is here: https://salsa.debian.org/release-team/release-tools/-/blob/master/mailer/mail_autoremovals.pl

IMO, if nobody has time to do it, the person who designed the tool should take care of fixing it. Instead of sending 536 mail, it should be sending a single mail with 536 entries in it... And it's not the first time I'm writing this."