Manifests of jars built with Gradle do not have a Implementation entries

[scottfrederick]

The Spring Boot Maven plugin configures the maven-jar-plugin to add the default Implementation-* entries to the built archive. Archives built with the Spring Boot Gradle plugin do not have these manifest attributes by default. The Gradle plugin should add the following manifest entries for consistency:

• Implementation-Title
• Implementation-Version
• Implementation-Vendor

This is similar to #32829, which we categorized as a bug and fixed in 3.0.x. We should fix this in the same version.

[wilkinsona]

Maven uses the following defaults:

Implementation-Title: ${project.name}
Implementation-Version: ${project.version}
Implementation-Vendor: ${project.organization.name}

project.name and project.version are straightforward with Gradle. project.organization.name is not. Maven will only add Implementation-Vendor if <organization>/<name> appears in the pom which, in my experience, isn't very common. I think we should just focus on the title and version.