make sure removed secret values are actually removed

[machisuji]

stringData is always merged with the data of the existing secret

with this explicit reset values removed from, say, the environment value will not be removed, which is unexpected

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: d97afc317cea0cd0720205913b841164cb4fb998

The changes in this PR will be included in the next version bump.

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| Name | Type | | ------------------------ | ----- | | @openproject/helm-charts | Minor |

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