docs: fix V2 artifacts and content issues in routing-requests-to-rest-apis.md

[Maanvi212006]

Describe your pull request here:

Fixes V2-era artifacts and content issues in routing-requests-to-rest-apis.md so the article accurately reflects the current V3 implementation (Spring Cloud Gateway) and no longer mixes routing concepts with HA deployment architecture.

Changes:

• Replaced "In addition to the basic Zuul routing" with "In addition to basic routing" — Zuul is V2-only and no longer part of the V3 gateway
• Updated example URLs to use the V3 default gateway port (7554) instead of the old V2 ports (10015/10010)
• Fixed typo: "ditributing" → "distributing"
• Removed the empty "Implementation details for routing" heading
• Condensed the "Zowe architecture with high availability enablement on Sysplex" section (Sysplex/LPAR/zowe.yaml/VSAM details) into a brief "High availability" summary with a link to the existing dedicated page, ../configure-sysplex.md, instead of duplicating that content here

Closes #5077

List the file(s) included in this PR:

• docs/user-guide/api-mediation/routing-requests-to-rest-apis.md

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[github-actions]

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