BE: Nuke spring logback config to enable default boot configs

[Haarolean]

• Breaking change? (if so, please describe the impact and migration path for existing application instances)

What changes did you make? (Give an overview)

Nuked configured logback config.

1. No idea why we had one in the first place, the log message doesn't differ much:
   before:
   2025-10-31 22:38:50,017 INFO [kafka-admin-client-thread | kafbat-ui-admin-1761925126-1] o.a.k.c.NetworkClient: [AdminClient clientId=kafbat-ui-admin-1761925126-1] Node -1 disconnected.
   after:
   2025-10-31T22:39:26.466+07:00 INFO 94452 --- [in-1761925163-1] org.apache.kafka.clients.NetworkClient : [AdminClient clientId=kafbat-ui-admin-1761925163-1] Node -1 disconnected.
2. Removing one allows spring boot to use default integration mechanisms and overrides via env vars, see Support JSON Logging capability #1171

Fixes #1171

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Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

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