Set default compressor to zstd-better-compression (#2954)

Will only be picked up by newly created repos. Existing repos will continue to use s2-default compressor.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

* closes #2840

#### Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/conn.go

@@ -24,7 +24,7 @@ import (
 const (
 	defaultKopiaConfigDir  = "/tmp/"
 	defaultKopiaConfigFile = "repository.config"
-	defaultCompressor      = "s2-default"
+	defaultCompressor      = "zstd-better-compression"
 	// Interval of 0 disables scheduling.
 	defaultSchedulingInterval = time.Second * 0
 )