Change from using "pode.<env>.json" files to "server.<env>.psd1" files

[Badgerati]

At the moment Pode uses pode.<env>.json files for settings and configuration.

For v1.0.0 this should be changed to normal PowerShell standard of using .psd1 files, and the name format altered to: server.<env>.psd1.

These files can be read in using Import-PowerShellDataFile -Path '<path>' on PS5+.

• Current rules using $env:PODE_ENVIRONMENT to determine which config file to use should remain the same.
• Settings and Config supported in the current format should also be supported in the new (unless another v1.0.0 issue drops it, but not for this issue).

[Badgerati]

This issue does also mean that PS3/4 will become unsupported from v1.0.0+. This was already going to happen due to the work on integrating Kestrel and the requirement for classes.

• The pode.psd1 should be updated to a minimum version of 5.0.
• The README/Documentation should be updated to remove the "PS3/4 Support".