Update the Beats packaging pipeline settings

[alexsapran]

Proposed commit message

This commits changes the settings of the packaging pipeline to make it execute only on the selected branches which currently is only main.

[elasticmachine]

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• Duration: 12 min 17 sec

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:robot: GitHub comments

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• /package : Generate the packages and run the E2E tests.

• /beats-tester : Run the installation tests with beats-tester.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[dliappis]

buildkite run docs-build