this also removes the reference to the long obsolete
https://hg.mozilla.org/mozilla-central/rev/061b97e02ede.

mozilla calls this 'esm-ification'. the last relevant to us modules were
ported in mozilla124[1], so this is our new minimum version. the old
version will likely be compatible with firefox up to version 128esr[2].

static imports are not supported in autoconfig scripts. i tried to keep
the diff as small as possible w.r.t comm-central's final JSMs. because
of this, we don't lazy-load any modules any more, nor provide any lazy
getters, as they would need to be loaded into a `lazy` object instead
the global (`this`) namespace, causing more churn. other than that, the
largest change was removing the now-useless Services.jsm workaround as
well as removing globalGetters for objects already loaded automatically.

[1]: https://hg.mozilla.org/mozilla-central/rev/68ba071ff6fb9978937496f9adc48e378957f594
[2]: bugzil.la/1881890

regressed-by: https://bugzil.la/920187

This is not necessary, as it is only checked in
XPIDatabase.jsm::mustSign(), which we can override instead, and in
aboutaddonsCommon.js::isDisabledUnsigned(), which we cannot affect
anyways and only uses it to show a red warning bar for legacy addons.

This also removes the old AddonSettings.ALLOW_LEGACY_EXTENSIONS
override, which was only used in Extension.jsm::experimentsAllowed()
and XPIDatabase.jsm::isDisabledLegacy().

This is in preparation for the ESM-ification endeavours going on at
Mozilla, which will make importing a module's global object (as we did
with `let Xdb = Cu.import(...)`) impossible, but was needed to get a
reference to AddonSettings (instead of a copy). Step two will be
replacing Cu.import() with ChromeUtils.import(), which has stricter
semantics about exported objects (doesn't hurt us; see Bug 1766114).

Having signatures enforced causes a big red error message to appear
below unsigned extensions, telling the user the extension has been
disabled. This is not true, though; the extensions are still enabled.
Setting xpinstall.signatures.required;false downgrades this error to a
(yellow) warning.

This broke installing some, but not all, addons. The reason is that some
addons derive their addon-id from a certificate's CommonName in
XPIInstall.jsm::loadManifest(aPackage, aLocation, aOldAddon)

regressed by: Bug 1524327