Unpublishing old EC2 AMIs

Hi everyone (in particular EC2 users),

FreeBSD is now publishing 34 EC2 AMIs every week: 2 (amd64,arm64) for
13.5-STABLE, and 16 each (amd64,arm64/ufs,zfs/base,builder,cloud-init,small)
for 14.3-STABLE and 15.0-CURRENT.  Across all the AWS regions, this adds up
to 5 TB of disk snapshots per week; while Amazon has been quite generous in
sponsoring FreeBSD's release engineering AWS account, I don't want to impose
unnecessary costs, especially since many of those weekly snapshot images never
get used.

I do plan on keeping:
* All of the *-RELEASE images, in all regions
* AMIs less than 2 months old, in all regions
* AMIs which have been launched in the past year (in the region where they
were launched),
* Every AMI in the us-west-2 and us-east-1 regions.

That last one is because it's not-infrequently useful to look at old images
to see when a bug was introduced or when behaviour changed; I'm keeping the
images in us-east-1 because that's the original (and for some purposes still
"default") region, and in us-west-2 because that's where a lot of new instance
types first become available (hence it's important for any pre-launch testing
I do).

At some point we might start reaping old GCE/Azure/Oracle/etc images as well,
but we don't publish nearly so many of those so I'm not in any great rush.

-- 
Colin Percival
FreeBSD Release Engineering Lead & EC2 platform maintainer
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid