Bump linkedin/iceberg 1.2 to 1.2.0.19 (NestedField column-default APIs)

[cbb330]

Picks up linkedin/iceberg#250 (released as com.linkedin.iceberg tag v1.2.0.19).

Summary

Bumps the Iceberg 1.2 line from 1.2.0.18 → 1.2.0.19. The only change in v1.2.0.19 is linkedin/iceberg#250, which re-baselines upstream's column default-value API (Types.NestedField initial/write defaults, Expressions.lit(), plus SchemaParser serialization) onto openhouse-1.2.0, bringing the 1.2 line to parity with the 1.5 line on this API.

• Defaults are stored as Literal<?> with withInitialDefault / withWriteDefault builders and matching accessors (initialDefault() / writeDefault() / *Literal()).
• SchemaParser serializes the defaults, so they are format-compatible end to end.
• v3-only bits (e.g. UnknownType) are dropped since 1.2 lacks them; everything else is a faithful copy from upstream main rather than a cherry-pick.

Changes

• Client-facing API Changes
• Internal API Changes
• Bug Fixes
• New Features
• Performance Improvements
• Code Style
• Refactoring
• Documentation
• Tests

Dependency version bump only — single line in build.gradle. The 1.5 line is already at its latest (1.5.2.15); this brings the 1.2 line to its latest within-line release.

Testing Done

• Manually Tested on local docker setup.
• Added new tests for the changes made.
• Updated existing tests to reflect the changes made.
• No tests added or updated. Tests for the new APIs live in [default values] Add NestedField column-default APIs and Expressions.lit() (~upstream #9502) iceberg#250; this PR is a version bump.
• Some other form of testing.

Additional Information

• Breaking Changes
• Deprecations