Well, I'd take this as a gift. I dislike forced jollity, team days are my idea of hell on earth, and cliques are anathema to me. (So maybe I am a curmudegeon, but the plus-side of that is that you'll never find me harming a colleague or engaging in toxic gossip about anyone else).
But I do know that not everyone is like me, and I well remember how isolating and unpleasant it was to be the one ostracised in this way when I was much younger and hadn't yet run out of fucks. Professionalism and basic courtesy are the minimum you should be entitled to expect. These colleagues are affording you neither, quite aside from their decision to socialise without you.
It sounds as though you're being managed out, and your workplace made so intolerable to you that you take the decision to move on. It's cowardly, childish, bullying behaviour and YANBU to be upset.
I'd arm yourself with some noise cancelling headphones, turn up and do your job, and comfort yourself with the knowledge that these are not people with whom you'd voluntarily socialise anyway.
It's not you. It's them. 🌹