Update docs in reaction to change DOTNET_ to ASPIRE_ in config variables

[DamianEdwards]

The environment variables used to configure various aspects of Aspire App Host projects and the Aspire Dashboard are being updated to remove the DOTNET_ prefix from them and replace it with ASPIRE_. In the case where the prefix was DOTNET_ASPIRE_ before, it's now ASPIRE_.

Note that the old environment variables will still work, but new ones should be the ones documented.

[DamianEdwards]

Note we shouldn't make this change live until the 9.2 version of the dashboard container is published.

[JamesNK]

The dashboard container isn't published at the same time as the rest of Aspire. We don't want to refer to new names in the docs when the latest bits don't support them. Therefor, it's best to not update anything until the latest stable version of the dashboard is 9.2.

I don't think we want to mention both names in the docs, just the latest names. However, we should probably have notes on these pages that the names have changed and 9.2 is required:

• https://learn.microsoft.com/en-us/dotnet/aspire/fundamentals/dashboard/configuration?tabs=bash
• https://learn.microsoft.com/en-us/dotnet/aspire/app-host/configuration

Note should also mention that we'll remove support for the older names at some point in the future, and you should update to use new names. We don't have a decided release to do that. Will probably happen Aspire 10 or 11 to give folks lots of time.