Fix outdated Gradle command in Maven Package Registry documentation

What does this MR do and why?

Updates the Gradle installation command from the deprecated gradle install to the current command gradle build.

Also: GitLabDuo highlighted that Install packages specified in your Gradle project. could be misleading. As suggested during the review, I changed it to Assembles and tests this project. to match the output of gradle help --task build:

Description
     Assembles and tests this project.

Problem

The current documentation instructs users to run gradle install to download and install Maven packages when using Gradle. However:

• The gradle install task does not exist by default in modern Gradle builds
• The legacy maven plugin provided the install task. It was deprecated in Gradle 6.0 and removed in Gradle 7.0
• Users following the documentation with the current Gradle version get Task 'install' not found errors when trying to follow the instructions

Notes

• The maven plugin that provided the gradle install command was deprecated in gradle 6.0, and removed in gradle 7.0 and later.
• gradle 6.0 was released November 8, 2019 - ~6 years ago
• The last gradle 6.x release is 6.9.3, in Oct. 17, 2022 - ~3 years ago
• gradle 7.0 was released April 9, 2021 - ~4 years ago

References

• https://gradle.org/releases/
• Deprecation in Gradle 6.0
• Removal in Gradle 7.0

Screenshots or screen recordings

NA

How to set up and validate locally

NA

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