feat: upgrade slf4j to 2.0.18 and replace logback with slf4j-simple (#905)

[Mikkey-f]

Closes #905

Upgrades the slf4j stack from 1.7.36 to 2.0.18 and replaces the test-time
logback binding with slf4j-simple, as discussed in the issue.

Changes

• slf4j-api, jcl-over-slf4j, log4j-over-slf4j: 1.7.36 → 2.0.18
• Test binding: logback-classic 1.2.13 → slf4j-simple 2.0.18
  (logback 1.2.x is incompatible with slf4j 2.x; logback 1.5.x requires
  Java 11, which conflicts with the project's JDK 8 CI matrix, and 1.3.x
  is end-of-life)
• fesod-sheet test logging config: logback.xml → simplelogger.properties
  (same console INFO output shape)
• log4j-to-slf4j 2.24.3 is untouched (versioned independently of slf4j)

Why low-risk

• No production code uses the slf4j API directly; the only consumer is POI,
  which runs fine on slf4j 2.x (backward compatible)
• fesod-shaded only relocates spring-core, so slf4j is untouched there
• The binding is test-scope only

Verification

• Full test suite: 887 tests, all green
• spotless:check passes