Update version of Datadog Gradle Plugin used in samples to 1.15.0

[0xnm]

What does this PR do?

This PR updates version of Datadog Gradle Plugin used in samples to 1.15.0.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.96%. Comparing base (dd70983) to head (861d568).
Report is 6 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #308   +/-   ##
==========================================
  Coverage      77.96%   77.96%           
- Complexity       258      260    +2     
==========================================
  Files             31       31           
  Lines            939      939           
  Branches         113      113           
==========================================
  Hits             732      732           
  Misses           151      151           
  Partials          56       56