[BUG][Java][client] jackson-databind-nullable version variable Gradle Incompatibility

Description

When generating a Java Client library, the jackson-databind-nullable version parameter (jackson-databind-nullable-version) used in build.gradle.mustache contains dashes instead of underscores. This results in an error during a Gradle build:

(((jackson - databind) - nullable) - version) is a binary expression, but it should be a variable expression

The dashes for this version parameter should be replaces with underscores. Example file with error

Gradle Version 5.3.1

openapi-generator version

OpenAPI Generator Version 4.1.2

Command line used for generation

  java8: true,
  dateLibrary  : "java8",
  useTags: true,
  library: "feign",
  feignVersion: "10.x"

Start with modules/openapi-generator/src/main/resources/Java/build.gradle.mustache, the example file named in the issue. Inspect the jackson-databind-nullable version parameter and update its naming as requested. Done means a generated Java client’s Gradle build accepts the version variable without the reported binary-expression error.