config: migrate strict_mode to Twig 3 compat flags for Grav 2.0

Grav 2.0's schema migration replaces the 1.7-era twig_compat with
twig2_compat/twig3_compat. Matching the repo to what the upgraded server
already generated so a content reset doesn't revert it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Np4cMQLF77i664CAQXySzU

config/system.yaml

@@ -237,7 +237,8 @@ flex:
       lifetime: 600
 strict_mode:
   yaml_compat: false
-  twig_compat: false
   blueprint_compat: false
+  twig2_compat: false
+  twig3_compat: true
 uploads_privacy:
   enabled: false