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

```diff @@ 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](https://app.codecov.io/gh/DataDog/dd-sdk-android-gradle-plugin/pull/245/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog)