chore(gradle): centralize repository config in settings.gradle.kts and simplify BOM management

### Summary
- Removed `repositories` blocks from individual `build.gradle.kts` files, moving configuration to `settings.gradle.kts`.
- Replaced custom Spring Boot constraints in the platform BOM with the Spring Boot BOM for cleaner dependency management.
- Explicitly added `webflux` dependency for Gateway to handle transitivity changes in Spring Boot 4.x.

backend/services/registry/oeps-importer/build.gradle.kts

@@ -5,9 +5,8 @@ plugins {
 group = "at.mocode"
 version = "1.0.0-SNAPSHOT"
 
-repositories {
-  mavenCentral()
-}
+// The 'repositories' block was removed from here.
+// Repository configuration is now centralized in 'settings.gradle.kts'.
 
 dependencies {
   testImplementation(platform("org.junit:junit-bom:5.10.0"))