Fix PowerShell-Support-Lifecycle.md: Current releases are supported for 6 and not 3 months after a subsequent Current or LTS release

[bergmeister]

PR Summary

As PowerShell's support is linked to the .NET support policy, current versions stay in support for 6 months: https://dotnet.microsoft.com/platform/support/policy/dotnet-core#cadence
This PR updates this historic artefact/left-over. the table here already uses 6 months as the policy.

PR Context

https://twitter.com/Steve_MSFT/status/1457775246698631175

Select the area of the Table of Contents containing the documents being changed.

Conceptual content

• Overview and Install
• Learning PowerShell
  • PowerShell 101
  • Deep dives
  • Sample scripts
  • Remoting
• Release notes (What's New)
• Windows PowerShell
  • WMF, ISE, release notes, etc.
• DSC articles
• Community resources
• Gallery articles
• Scripting and development
  • Language Spec
  • Legacy SDK

Cmdlet reference & about_ topics

• Preview content
• Version 7.2 content
• Version 7.1 content
• Version 7.0 content
• Version 5.1 content

PR Checklist

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• PR is targeted at the staging branch
• All relevant versions updated
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[opbld31]

Docs Build status updates of commit a919112:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/install/PowerShell-Support-Lifecycle.md	✅Succeeded	View (>=powershell-5.1)

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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[TravisEz13]

I don't think we should do this for 7.1 as people may have planned based on the current lifecycle.. So, perhaps say after 7.1.

[bergmeister]

If that's the case then this table here would need to be updated instead as it uses 6 month of support for 7.1

[TravisEz13]

I'm saying the table would stay the same, only future version would be affected by the policy, e.g. 7.3.

[sdwheeler]

@SteveL-MSFT said that we should be following .NET. And their policy is 6 months.