Reintroduce version 5 support or clarify that it was replaced by the version 7 prerelease

[connor-roehricht]

Observation

1. https://github.com/PowerShell/PowerShellStandard/tree/master/src/5 contains only references to 7 (note that the folder is still named 5).
2. https://github.com/PowerShell/PowerShellStandard/blob/master/src/dotnetTemplate/README.md contains a sample help text that only mentions 3 and 7.
3. This is inconsistent with the content of https://github.com/PowerShell/PowerShellStandard/blob/master/src/dotnetTemplate/Microsoft.PowerShell.Standard.Module.Template/Microsoft.PowerShell.Standard.Module.Template/.template.config/template.json , which is why the template currently does not work (besides Template for 7.0.0-preview.1 in NuGet.org seems to be broken #81)

Proposed Solutions

Option A

Reintroduce version 5 support by correcting the changes made in c7c7ad3 . #87 and #88 are then obsolete.
This seems to be preferred: #88 (comment)

Option B

Clarify that only 3 and 7 are supported now by removing references to 5 (could be done by renaming the aforementioned folder + merging of #88)