Mark mozjs78 as having limited security support

mozjs78 is the version of SpiderMonkey that will be in bullseye. It's in the same situation as mozjs68 and earlier versions: it has upstream security support right now, but the upstream security support will finish part way through the lifetime of bullseye, and updates are slow to prepare due to its large size. The package description specifically calls it out as unsuitable for untrusted scripts.