Update aws-batch.mdx External ID for Cloud

[MichaelTansiniSeqera]

DRAFT improve external ID

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[MichaelTansiniSeqera]

Adding reviewers, specifically @munishchouhan for correctness and @gwright99 for technical clarity

[stefanoboriero]

We have 3 ways AWS credentials can be defined:

1. With AWS user access keys
2. With an AWS role and AWS user access keys to be used to assume the role (so the role is still assumed by the customer AWS user)
3. With an AWS role and externalID + trust policy for the role to be assumed by Seqera jump role

This change effectively removes the documentation of option number 2, driving people who want to user an AWS role towards the externalID option: I wanted to check if this is intentional. Feels a bit off removing documentation for something that is still supported by Platform and not deprecated AFAIK

[MichaelTansiniSeqera]

With an AWS role and AWS user access keys to be used to assume the role (so the role is still assumed by the customer AWS user)

@stefanoboriero no we should still include this but perhaps put it within its own section to make clear it's a hybrid role - I will amend.