The platforms flag should be single-valued

### Description of the feature request:

Right now, the `--platforms` flag accepts multiple values but only the first is actually used.

Instead, the flag should be single-valued and give an error if multiple values are passed.

Further, the flag should actually be `--platform` and uses should be migrated, but that may be harder to manage.

### Which category does this issue belong to?

Configurability

### What underlying problem are you trying to solve with this feature?

Poor decision decisions at the start of the platforms project

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse master; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_