Simplify AWS terminology in section headings

[msarage-ux]

Summary

This PR simplifies the terminology used in section headings by replacing the verbose "Amazon Web Services (AWS)" with the concise "AWS" abbreviation, aligning with recent documentation style improvements.

Changes

• modules/rosa-hcp-firewall-prerequisites.adoc: Changed heading from "Domains for Amazon Web Services (AWS) APIs" to "Domains for AWS APIs"
• modules/ossm-federation-across-cluster.adoc: Changed heading from "Exposing the federation ingress on Amazon Web Services (AWS)" to "Exposing the federation ingress on AWS"

Justification

Recent PRs have been standardizing terminology by using concise "AWS" in section headings while keeping the full "Amazon Web Services (AWS)" on first mention in body text. This change follows that established pattern for consistency across the documentation.

Testing

• Verified AsciiDoc syntax is correct
• Changes are limited to section headings only
• No functional impact

🤖 Generated with Claude Code

[openshift-ci]

Hi @msarage-ux. Thanks for your PR.

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