Mahout-592: Update PULL_REQUEST_TEMPLATE to align with current workflow

[guan404ming]

Purpose of PR

• Clarified that PR title format is only needed "if related issue exists"
• Consolidated testing/verification section
• Made the template more concise while maintaining all essential checks

Linked Issues

• closes Update PULL_REQUEST_TEMPLATE to align with current workflow #592

Changes Made

• Bug fix
• New feature
• Documentation update

Important ToDos

Please mark each with an "x"

A GitHub issue exists (if not, please create one) [https://github.com/apache/mahout/issues]

• Title of PR is "Issue #XXXX: Brief Description of Changes" where XXXX is the GitHub issue number.
• Created unit tests where appropriate
• Added correct licenses on newly added files
• Assigned GitHub issue to self
• Added documentation in ScalaDocs/JavaDocs and to the website
• Successfully built and ran all unit tests, verified that all tests pass locally

If all of these items are not yet complete, but you still feel it is appropriate to open a PR, please open it as a Draft PR instead.
Once all requirements are met, you can mark it as ready for review.

Breaking Changes

Does this PR introduce a breaking change?

• Yes
• No

Testing & Verification

Describe how you tested the changes.

• Unit tests added
• Manually tested

Checklist

• The title follows the format "MAHOUT-XXXX Brief Description"
• GitHub issue is created
• Code follows ASF guidelines

[andrewmusselman]

Looks good to me