feat(cache-control): [MC-30] Change max-age to 900

[mmiermans]

Goal

Match the cache-control duration of Firefox API Proxy to the legacy endpoint, which used 900 seconds. A lower cache duration can result in a higher click-through rate, because our feedback loop is faster.

References

JIRA ticket:

• MC-30

Endpoints:

• old Firefox recommendation endpoint responds with cache-control: s-maxage=900
• Firefox Proxy API responds with responds with cache-control: public,max-age=1800,public