Backport of When running under systemd, send ready when server completed reloading config #7028 into release/1.8.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #15041 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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Server must call SdNotifyReady when it completed reloading. Otherwise systemd gets timeout after vault service reload as mentioned in #7028.

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