[Release] update version to next minor 8.15.0

[elasticmachine]

Update version to next minor 8.15.0.

Merge after create the new release branch.

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @elasticmachine? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2024-04-18T12:50:04.120+0000

• Duration: 112 min 25 sec

Test stats 🧪

Test	Results
Failed	0
Passed	29390
Skipped	2061
Total	31451

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