Update docs for new compute environment default image

The default changed to Amazon Linux 2023 in January 2026.¹ Things are
largely the same and I don't think anything in the doc needs to change
given our usage (though I haven't actually tested the steps in this
section).

One small noteworthy change is replacement of YUM with DNF. I did not
change the `yum` command because AL2023 still provides `yum` as a
pointer to `dnf`.²

¹
https://docs.aws.amazon.com/batch/latest/userguide/ecs-migration-2023.html
² https://docs.aws.amazon.com/linux/al2023/ug/package-management.html

Author: victorlin

doc/aws-batch.md

@@ -364,11 +364,12 @@ but Amazon's prorated billing uses a minimum duration of one month.
 
 ### Disk space for your jobs
 
-_The following applies to Batch compute environments using Amazon Linux 2 (AL2)
-[ECS-optimized AMIs][], which is the default for new compute environments.  If
-you're using older compute environments with Amazon Linux 1 (AL1) AMIs, either
-upgrade or see previous versions of this document.  If you're using custom
-AMIs, you can probably find your own way._
+_The following applies to Batch compute environments using Amazon Linux 2023
+(AL2023) [ECS-optimized AMIs][], which is the default for new compute
+environments.  If you're using older compute environments with Amazon Linux 2
+(AL2) or Amazon Linux 1 (AL1) AMIs, either upgrade or see previous versions of
+this document.  If you're using custom AMIs, you can probably find your own
+way._
 
 By default, your Batch jobs will have access to ~28 GiB of shared space.  This
 is enough for many Nextstrain builds, but the [SARS-CoV-2 build][] is the
@@ -453,7 +454,7 @@ following user data blob to your launch template to configure them:
             "$0" init
     fi
 
-    yum install -y nvme-cli jq lvm2
+    dnf install -y nvme-cli jq lvm2
 
     declare -a instance_devices ebs_devices