Migrate from the deprecated `com.squareup:javapoet` to `com.palantir.javapoet:javapoet`

[Square's JavaPoet](https://github.com/square/javapoet) has been deprecated starting 2020-10-10 with the repo being achieved. Migrate DGS Codegen from the deprecated `com.squareup:javapoet` to `com.palantir.javapoet:javapoet`: https://github.com/square/javapoet#deprecated.