Bump `slf4j-api` version

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Motivation

Fluss uses an outdated `sfl4j-api` version (1.7.36).
This limits users to old `logback` versions (1.2.x and below).

### Solution

- Bump to latest `slf4j-api` version
- Remove info about compatible logback versions [in docs](https://alibaba.github.io/fluss-docs/docs/maintenance/observability/logging/#configuring-logback)
- Update observability quickstart guide to use the newest logback version

### Anything else?

Blocked by #615

### Willingness to contribute

- [x] I'm willing to submit a PR!