PR Summary

Update the feature to support the notification type that can be configured using the environment variable POWERSHELL_UPDATECHECK.
This is to address the feedback from powershell committee in the RFC review: PowerShell/PowerShell-RFC#162 (comment), quoted below:

Coming out of @PowerShell/powershell-committee review, we want to change the environment variable to no longer be a simple opt-out/in but rather a field that can be used to set the "channel". So e.g. a $env:PSNotificationPreference that can be set to Default, Off, or LTS, where the latter would only provide LTS update notifications in their notifications.

POWERSHELL_UPDATECHECK accepts 3 values: Off, Default, and LTS (case-insensitive), and the behavior is as follows:

• Off: turn off the udpate notification.
• Default: give you the default behaviors
  • the preview version 'pwsh' checks for the new preview version and the new GA version.
  • the GA version 'pwsh' checks for the new GA version only.
• LTS: both preview and GA version 'pwsh' checks for the new LTS version only.

For a specific version of pwsh and a specific notification type, only one update check task, at most, will run to complete per a day. Other tasks should be able to detect "a check is in progress" or "the check has been done for today" and bail out early, to avoid any unnecessary network IO or CPU cycles.

Note that this is a breaking change comparing to preview-5 and preview-6, because the environment variable name and its accepted values are changed.

PR Context

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