Drop containerd-2.1

[Sparksssj]

Description of changes:

• Drop containerd-2.1 now that all variants have been moved to containerd-2.2
• Add end-of-support BRSA advisory (advisories/staging/BRSA-zosf9tq7lefr.toml) for containerd-2.1

Note: This depends on bottlerocket-os/bottlerocket#4869 (moves all variants to containerd-2.2, including aws-ecs-4/aws-ecs-4-nvidia) landing first. Removing containerd-2.1 before all variants are on containerd-2.2 would break those variant builds.

Testing done:

• bottlerocket-core-kit builds for both arches (x86_64, aarch64); validate-kits passes and the built kit contains only containerd-1.7 and containerd-2.2 RPMs (no containerd-2.1)
• Published the kit to a personal ECR and built all AWS variants against it (with #4869's variant changes applied) — all 22 AMI-producing variants build cleanly on both arches (44 AMIs), confirming no variant depends on containerd-2.1

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.