Define what minimum version of the spec we target

For things like https://github.com/vector-im/element-web/issues/22337 and similar the fix is to use the stable endpoint, but at what point do we consider it safe to drop the version check around an HTTP call? In the general case, we drop the version check once "sufficient users" are on a compatible homeserver, however with the spec's new versioning scheme it's hard to determine when that would be exactly.

The spec's new versioning scheme also makes the question a bit awkward, though this will be looked at in https://github.com/vector-im/element-web/issues/16876