Install scripts: remove support for alpha and beta versions

[ghost]

Currently, both the Powershell and the POSIX script support standalone installations with early alpha and beta releases, which needs some extra code, for example:

https://github.com/opentofu/get.opentofu.org/blob/main/src/install-opentofu.ps1#L325 https://github.com/opentofu/get.opentofu.org/blob/main/src/install-opentofu.sh#L748 https://github.com/opentofu/get.opentofu.org/blob/main/src/install-opentofu.sh#L831

However, we should explicitly warn users that these versions cannot be installed using the script to avoid confusion.

As 1.6.0 is stable, it is unlikely that someone would want to install these early versions. In addition, alpha versions don't work with the current registry anymore.

Note: we are not removing the early binaries, merely the support for installing them via an install script. You can still download any of the early binaries from the releases page.

[jjshanks]

I can work on this

[ghost]

@jjshanks heads up, this may take a while to get merged depending on when we want to drop support. ~Also, please base it on #11 since that needs to get merged before this.~

[ghost]

Fixed in #18