Based on standard practices, we need to have a copyright and license notice at the top of each source file. Removed existing copyrights and updated/added copyright notices for .h, .cpp, .cs, .ps1, and .psm1 files.
Updated module manifests for consistency to have Author = "PowerShell" and Company = "Microsoft Corporation". Removed multiple line breaks.
Separate PR coming to update contribution document for new source files: #6140
Manually reviewed each change.
Fix#6073
- Add 'ArgumentCompletionsAttribute' to support argument completion for parameters that cannot have a ValidateSetAttribute.
- Use 'ArgumentCompletionsAttribute' for the '-Format' parameter of 'Get-Date' to enable useful argument compeltions.
