Update JSLink migration guide and user custom actions ecb migration guide

[Ashlesha-MSFT]

Category

• Content fix

Related issues

• fixes SPFx Docs Update for content : migrate-jslink-to-spfx-extensions.md #10684
• fixes SPFx Docs Update for content : migrate-user-customactions-to-spfx-extensions.md #10683

What's in this Pull Request?

User Custom Actions migration guide - Updated with modern references
JSLink migration guide - Updated with modern references

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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[learn-build-service-prod]

Learn Build status updates of commit 1f3daa5:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/spfx/extensions/guidance/migrate-jslink-to-spfx-extensions.md	✅Succeeded	View
docs/spfx/extensions/guidance/migrate-user-customactions-to-spfx-extensions.md	✅Succeeded	View

For more details, please refer to the build report.