Update Sentinel cost workbook

[Dr Bill Mcilhargey (billmcilhargey)]

Summary

• Update the SOC Handbook AzureSentinelCost workbook with reorganized cost sections and updated benefit guidance.
• Expand the Defender for Servers Plan 2 eligible data-source description to mirror the Microsoft 365 benefit section style.
• Add the SOC Handbook release note for the packaged workbook enhancements.

Scope

This PR intentionally includes only:

• Solutions/SOC Handbook/Workbooks/AzureSentinelCost.json
• Solutions/SOC Handbook/ReleaseNotes.md

Other JSON changes from the source branch are excluded.

Validation

• Parsed Solutions/SOC Handbook/Workbooks/AzureSentinelCost.json as JSON successfully.
• Verified the branch diff against master contains only the two scoped files.
• Ran git diff --check on the scoped files successfully.

[Dr Bill Mcilhargey (billmcilhargey)]

Superseded by #14998. Its scoped SOC Handbook workbook and release-note content was already identical in #14998; the complete #15020 summary, scope, and validation record have been retained in #14998.