Use Datadog Gradle Plugin 1.13.1 for samples

[0xnm]

What does this PR do?

This PR updates Datadog Gradle Plugin version used for the sample files to 1.13.1.

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

Merging #245 (f56c831) into develop (9aabde7) will increase coverage by 0.13%.
The diff coverage is n/a.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #245      +/-   ##
=============================================
+ Coverage      81.08%   81.21%   +0.13%     
- Complexity       201      202       +1     
=============================================
  Files             26       26              
  Lines            793      793              
  Branches         104      104              
=============================================
+ Hits             643      644       +1     
  Misses           112      112              
+ Partials          38       37       -1     

see 1 file with indirect coverage changes