`tiup playground lts` to run the latest LTS without knowing the version

Feature Request

Is your feature request related to a problem? Please describe:
I want tiup playground have an option to startup using the latest LTS version, not typing the exact LTS version like v6.1.5 or v6.5.0. Currently, without specifying the [version] argument, it startup the latest TiDB version regardless they are LTS or DMR. It is hard to remember which version is LTS or DMR and some users just want to run the latest LTS.

Describe the feature you'd like:

tiup playground lts starts up the latest LTS version as nightly runs the daily build ones. I do not need dmr option because tiup playground just runs the latest version.

Why the featue is needed:

It is hard for a lot of users to remember which version is LTS or DMR. Also if users want to run TiDB cluster in production, they want to test the features using the LTS version not DMR.

Describe alternatives you've considered:

Remember the LTS version and type the version number exactly like tiup playground v6.5.0

Teachability, Documentation, Adoption, Migration Strategy:

Start at the tiup playground command entry point and compare its version handling with the existing nightly behavior. Determine how the latest LTS version can be selected, then verify that tiup playground lts starts that version while existing version and default behavior remain unchanged.