Kotlin 2.0.21 & AGP 8.6.1

[0xnm]

What does this PR do?

The main point of this PR is to have Kotlin updated to 2.0.21: this allows Xcode 16 usage.

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)