Rename validatePlugins to validatePluginsSkipkit to avoid task name clash with Gradle 6.x

[vlsi]

This fixes #876 (GradlePortalReleasePlugin: Cannot add task 'validatePlugins' as a task with that name already exists)

[vlsi]

ShipkitJavaIntegTest fails with Gradle 6.5, and the cause seems to be tasks returned in the different order.

I don't know what does the test verify, so I don't know how to heal it.

@mockitoguy , can you please clarify?

Expected:

...
:api:generatePomFileForJavaLibraryPublication
:api:compileJava
:api:processResources
:api:classes
:api:jar
:api:javadoc
:api:javadocJar
...

Actual:

...
:api:compileJava
:api:processResources
:api:classes
:api:jar
:api:generateMetadataFileForJavaLibraryPublication
:api:generatePomFileForJavaLibraryPublication
...

[vlsi]

One of the workarounds could be "ignore :generatePom and :generateMetadata" tasks in skippedTaskPathsGradleBugWorkaround. The tasks do not seem to be important much, and they seem to be the only ones that produce unordered results.

WDYT?

[mockitoguy]

@vlsi, we are going to put the project in maintenance mode soon. The new plugins are:

• https://github.com/shipkit/shipkit-changelog
• https://github.com/shipkit/shipkit-auto-version

Check out the new plugins. Would they work for you?