Replace ktlint with Spotless for code formatting

- Add Spotless plugin 8.1.0 with ktlint 1.7.1 and Google Java Format
- Configure ktlint rules, disable filename/max-line-length/property-naming
- Remove old ktlint plugin

gradle/libs.versions.toml

@@ -0,0 +1,6 @@
+[versions]
+spotless = "8.1.0"
+ktlint = "1.7.1"
+
+[plugins]
+spotless = { id = "com.diffplug.spotless", version.ref = "spotless" }