TL;DR, it's Lightsaber not Lightsabre.
Usage in canon
Outside of a couple of typographical errors in the Phantom Menace script, the sole usage is lightsaber in every single canon article on the databank, every single canon novel and every single factbook. It's also used exclusively as lightsaber in every single EU novel, gamebook and script book.
So where can we find the term lightsabre?
StarWars.com highlights a couple of instances where the term lightsabre has been used in various (now) non-canon sources, including a glossary in the Star Wars Official Collector’s Edition (1977) published by Marvel. The writer of the article notes that it's been spelled incorrectly, along with other canon changes.
Back then we wrote “Artoo Detoo” and “See Threepio” instead of R2-D2
and C-3PO, Ben Kenobi used a “lightsabre,” not a lightsaber, and
Kenobi, Obi-Wan was the one-time leader of the Jedi Knights. Certainly
a lot has changed, but there was a special kind of magic to the galaxy
when the only story we had was the original Star Wars.
What Trademarks does LucasFilm own?
LucasFilm own and closely guard the term lightsaber, entering into trademark disputes with no less than a dozen companies that have dared to use the word in their branding. By comparison, there's a sole entry in the US and EU trademark database for lightsabre held by a company called Powerbox AG. Lucasfilm haven't disputed the use of lightsabre.
In short, Lucasfilm and Disney have never knowingly used the term lightsabre. The sole usage comes from a few minor instances in articles and magazines from non-canon sources such as licensed works, fan magazines and Marvel magazines.
These terms are not interchangeable and we should correct this common misspelling wherever we see it.