Fixes for Kotlin 2.1 in codegen_cli

Replace `toLowerCase()` with `lowercase`.

Replace `appendln()` with `appendLine()`.

Bug: 399463072
Flag: EXEMPT minor refactor
Test: m out/soong/.intermediates/frameworks/base/tools/codegen/codegen_cli/linux_glibc_common/kotlin/codegen_cli.jar
Change-Id: Iab0e484380d65f7b764c74c26bc592e65c567f0e