fix(scoop-config): Output [DateTime] as [String]

[niheaven]

Description

Beginning in PowerShell 6, ConvertTo-Json attempts to convert strings formatted as timestamps to DateTime values.

https://learn.microsoft.com/powershell/module/microsoft.powershell.utility/convertfrom-json?view=powershell-7.2#notes

So this fix coerce [DataTime] output to [String] to make uniform output in PS5 and PS7.

Motivation and Context

• Closes [Bug] Different output for scoop config last_update depending on shell #5229

How Has This Been Tested?

PS5:

❯ scoop config last_update
2022-10-30T17:14:47.9357171+08:00

PS7 (before):

❯ scoop config last_update

2022年10月30日 17:14:47

PS7 (after):

❯ scoop config last_update
2022-10-30T17:14:47.9357171+08:00

Checklist:

• I have read the Contributing Guide.
• I have ensured that I am targeting the develop branch.
• I have updated the documentation accordingly.
• I have updated the tests accordingly.
• I have added an entry in the CHANGELOG.