Upgrade to Spring Boot 4

[timonback]

Action items and breaking changes for Spring Boot 4:

• Migrate to Spring Boot 4 -> Chore/migrate to spring boot 4 #1550
  • Migrate fully to Swagger 3 (currently swagger 2 still included due to swagger-core)
• Resolve Use OpenAPI 3.1 fields #1487
• Resolve UI publish endpoint fails when spring.jackson.property-naming-strategy=SNAKE_CASE (payload fields generated in camelCase) #1535 -> fix(core): allow to publish with different jackson naming strategies #1563
• Address maintainer notes in source code comments
  • Make field servers optional -> feat(core): allow empty servers configuration #1572
  • Use field type internally as array -> chore: resolve some maintainer notes with breaking changes #1571
  • Upgrade json-schema to: draft-07 -> align with https://github.com/asyncapi/spec-json-schemas/blob/master/schemas/3.0.0.json#L3
  • Do grouping query parameter instead of path?
  • Spell AsyncAPI everywhere with the official spelling
• Rename branch master to main -> chore: migrate to branch main #1551