Decide how to deal with upstream feature gates in a RR world

Rebasing on nightlies allows to have a smooth transition when it's time to get to the actual ESR.

However, NIGHTLY_END-based versions have some feature gates that we won't want, such as NIGHTLY_BUILD and EARLY_BETA_OR_EARLIER.

We need to decide whether we want to hardcode them to False in build/moz.configure/init.configure, or whether we want to rebase on release tags.

Using release tags means that alphas have Firefox quality, but they make it harder to switch to the ESR, as we'll have to do something like 151 release (tagged 2026-05-04) -> 152_nightly_end (tagged 2026-05-11, so only one week later) -> 153_nightly_end (tagged 2026-06-08) to gain a month (153-build1 will be tagged on 2026-07-06, but the majority of developments will be available one month earlier).