More clearly define when ESU license becomes active

[4rm]

Instead of "now", indicate that an ESU license can only be activated once regular support for Win10 ends, and ESU coverage begins.

Why

• We have users requesting ESU licenses to be applied before the coverage start date, because the KB says they can be applied "now". Other KBs accurately describe when the license can be applied.

Changes

• Replace "now" with more accurate indicator of when license can be applied: "once the defined ESU coverage period begins"

[learn-build-service-prod]

Learn Build status updates of commit a1aef05:

✅ Validation status: passed

File	Status	Preview URL	Details
windows/whats-new/enable-extended-security-updates.md	✅Succeeded

For more details, please refer to the build report.

[learn-build-service-prod]

Learn Build status updates of commit 9570bd9:

✅ Validation status: passed

File	Status	Preview URL	Details
windows/whats-new/enable-extended-security-updates.md	✅Succeeded

For more details, please refer to the build report.