Update canonical/data-platform-workflows action to v21

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
canonical/data-platform-workflows	action	major	v20.0.2 -> v21.0.0

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Release Notes

canonical/data-platform-workflows (canonical/data-platform-workflows)

### [`v21.0.0`](https://togithub.com/canonical/data-platform-workflows/releases/tag/v21.0.0) [Compare Source](https://togithub.com/canonical/data-platform-workflows/compare/v20.0.3...v21.0.0) ##### What's Changed - breaking(integration_test_charm.yaml): Only run juju 3.6 tests on `schedule` by [@​carlcsaposs-canonical](https://togithub.com/carlcsaposs-canonical) in [https://github.com/canonical/data-platform-workflows/pull/232](https://togithub.com/canonical/data-platform-workflows/pull/232) **Full Changelog**: https://github.com/canonical/data-platform-workflows/compare/v20...v21.0.0 ### [`v20.0.3`](https://togithub.com/canonical/data-platform-workflows/releases/tag/v20.0.3) [Compare Source](https://togithub.com/canonical/data-platform-workflows/compare/v20.0.2...v20.0.3) ##### What's Changed - patch(integration_test_charm.yaml): Add missing timeout-minutes by [@​carlcsaposs-canonical](https://togithub.com/carlcsaposs-canonical) in [https://github.com/canonical/data-platform-workflows/pull/231](https://togithub.com/canonical/data-platform-workflows/pull/231) **Full Changelog**: https://github.com/canonical/data-platform-workflows/compare/v20.0.2...v20.0.3

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