Convert DateTime to string in setReleaseTag

[iSazonov]

PR Summary

Related to #11126 and #11207

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• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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[daxian-dbw]

The purpose is to get the precision up to second, like 2019-12-03T22:04:15Z. ToString("o") gives you 2019-12-03T22:04:15.0000000Z.

We don't need to address this now, as the preview.json, stable.json and lts.json will have the ReleaseDate re-set at the end of the release pipeline, where pwsh is used.
I think we need to make the build agent pre-install pwsh eventually.

[TravisEz13]

@iSazonov 7.0.0-rc.1 is used for PRs that are already merged into the rc.1 branch. Please do not use this milestone. Perhaps you should let one of the maintainers assign the milestone for you.