ci: use new entity analytics page flag in EC test env

[gurevichdmitry]

Summary

• Replace the outdated Entity Analytics Kibana user_settings_yaml in the EC terraform module (used by the create-env EA workflow) with feature_flags.overrides: securitySolution.useNewEntityAnalyticsPage: true.
• The previous experimental flags change every release and are no longer relevant for 9.6.

Test plan

• Run the create environment EA workflow on a 9.6 stack and confirm the new Entity Analytics page is enabled

🤖 Generated with Claude Code

[mergify]

This pull request does not have a backport label. Could you fix it @gurevichdmitry? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v./d./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[mergify]

This pull request does not currently match the merge queue conditions, so it cannot be queued from here. The box comes back if it matches again.