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

[iuliiasobolevska]

Square's 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.

[iuliiasobolevska]

Feature requests that would make migration easier: palantir/javapoet#210, palantir/javapoet#211.