Summary

• add almalinux8 as a distinct supported operating-system value
• detect AlmaLinux 8 in ImageBuilder and preserve its release packages during kernel updates
• add schema, validator, configure, ImageBuilder, and integration-fixture coverage
• reject EFA-enabled AlmaLinux 8 configurations because the current EFA installer does not support AlmaLinux 8

Scope

This supports customer-provided AlmaLinux 8 parent images. It does not claim that AWS ParallelCluster publishes an AlmaLinux image.

Closes #7482.

Companion cookbook PR: aws/aws-parallelcluster-cookbook#3222

Tests

• 755 focused schema, validator, configure, and ImageBuilder tests passed
• AlmaLinux EFA validation covers x86_64, arm64, and EFA-disabled configurations
• an ImageBuilder build from an official AlmaLinux 8 x86_64 parent completed, and the resulting custom AMI passed the ImageBuilder boot/test phase
• changed ImageBuilder components and generic AlmaLinux fixtures passed diff and leak checks