Built-in https-everywhere storage is not migrated to idb

In #32017 (closed), we flipped the pref for using the new indexdb storage backend for webextensions. However, at the same time, we moved https-everywhere into the omni.ja with tor-browser-build#40073 (closed). We received a report that these two sets of changes conflict (tor-browser-build#40073 (closed) was accompanied by a few others), and https-everywhere storage is not migrated, as a result. These changed were included in 10.0a7.

Interestingly, in 10.0a7, I have a pref extensions.webextensions.ExtensionStorageIDB.migrated.https-everywhere-eff@eff.org. However, the reporter describes that the data in the old backend (Browser/TorBrowser/Data/Browser/profile.default/browser-extension-data/https-everywhere-eff@eff.org/storage.js) is not migrated to the data in the new backend (Browser/TorBrowser/Data/Browser/profile.default/storage/default/), and https-everywhere re-downloads missing data.