Add StartPage safe search option

### Prerequisites

- [X] I have checked the [Wiki](https://github.com/AdguardTeam/AdGuardHome/wiki) and [Discussions](https://github.com/AdguardTeam/AdGuardHome/discussions) and found no answer

- [X] I have searched other issues and found no duplicates

- [X] I want to request a feature or enhancement and not ask a question

### The problem

StartPage recently add safe search option https://support.startpage.com/hc/en-us/articles/31709891174420-Whitelisting-Startpage-at-the-DNS-level-safe-startpage-com through DNS mapping from startpage.com to safe.startpage.com, currently they're not listed in the safe search option of AGH

### Proposed solution

Add StartPage safe search to the option.

### Alternatives considered and additional information

_No response_