Promote `web-login` to stable.

[jastang]

Description of your changes

Promote up alpha web-login to stable maturity. This also keeps the alpha variant for at least one release before deprecating it to not break users/automation that may still be using it, but will hide it from up alpha.

How has this code been tested

Hidden from alpha but still useable:

./up alpha -h
Usage: up alpha <command>

Alpha features. Commands may be removed in future releases.

[...]

Commands:
  alpha upbound      Interact with Upbound.
  alpha xpkg         Interact with UXP packages.
  alpha migration    Migrate control planes to Upbound Managed Control Planes.
  alpha space        Interact with Spaces.
  alpha ctx          Select an Upbound kubeconfig context.

  [...]

 ./up alpha web-login 
jastang logged in

Help

  ./up -h     
Usage: up <command>

The Upbound CLI

Flags:
  -h, --help                Show context-sensitive help.
      --format="default"    Format for get/list commands. Can be: json, yaml, default
  -q, --quiet               Suppress all output.
      --pretty              Pretty print output.

Commands:
  license                Print Up license information.
  help                   Show help.
  login                  Login to Upbound.
  logout                 Logout of Upbound.
  web-login              Use web browser to login to up cli.
[...]

Usage

./up web-login
jastang logged in

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.