Document valid TZ is required for cron

### Describe the Bug

```
CronError: ERROR: You specified an invalid date.
at CronTime.getNextDateFrom (/directus/node_modules/.pnpm/cron@4.3.4/node_modules/cron/dist/time.js:130:19)
at CronTime.sendAt (/directus/node_modules/.pnpm/cron@4.3.4/node_modules/cron/dist/time.js:86:35)
at CronTime.getTimeout (/directus/node_modules/.pnpm/cron@4.3.4/node_modules/cron/dist/time.js:99:21)
at CronJob.start (/directus/node_modules/.pnpm/cron@4.3.4/node_modules/cron/dist/job.js:163:37)
at new CronJob (/directus/node_modules/.pnpm/cron@4.3.4/node_modules/cron/dist/job.js:68:18)
at CronJob.from (/directus/node_modules/.pnpm/cron@4.3.4/node_modules/cron/dist/job.js:81:20)
at scheduleSynchronizedJob (file:///directus/node_modules/.pnpm/@directus+api@file+api_@types+node@24.9.1_jiti@2.6.1_typescript@5.9.3/node_modules/@directus/api/dist/utils/schedule.js:8:25)
at schedule (file:///directus/node_modules/.pnpm/@directus+api@file+api_@types+node@24.9.1_jiti@2.6.1_typescript@5.9.3/node_modules/@directus/api/dist/schedules/telemetry.js:22:5)
at createApp (file:///directus/node_modules/.pnpm/@directus+api@file+api_@types+node@24.9.1_jiti@2.6.1_typescript@5.9.3/node_modules/@directus/api/dist/app.js:263:11)
at async createServer (file:///directus/node_modules/.pnpm/@directus+api@file+api_@types+node@24.9.1_jiti@2.6.1_typescript@5.9.3/node_modules/@directus/api/dist/server.js:23:38)
2025-12-02T17:10:33: PM2 log: App name:directus id:0 disconnected
2025-12-02T17:10:33: PM2 log: App [directus:0] exited with code [1] via signal [SIGINT]
```

### To Reproduce

Ive just upgraded.
I have a very basic directus running. (with postgres)

### Directus Version

v11.13.X

### Hosting Strategy

Self-Hosted (Docker Image)

### Database

postgres