Deprecate pex_binary.platforms in favor of complete_platforms

[benjyw]

The platforms field is a legacy hack: it specifies abbreviated platform strings that elide the full marker environment and compatibility tag information, causing spurious resolve errors. This leads to neverending user issues and support requests.

To remedy this we should:

• [x] Ensure obvious documentation of how to generate values for complete_platforms, and remove handwritten documentation of platforms. (Done in #20008)
• [x] Deprecate platforms and require use of complete_platforms.
• [ ] Warn if the pex targets multiple interpreter versions, but the --python-shebang constrains to just one of them.

We probably also want to tackle #15234 to remove one unnecessary hoop to specifying complete_platforms, but it's not a blocker.

[huonw]

There's at least one more place that is semi-equivalent to platforms: the FaaS target's runtime value will be converted to a platforms argument if there's no matching built-in complete_platforms.

https://github.com/pantsbuild/pants/blob/2e5c50511f809aa3c007bb54924277c3b0348f64/src/python/pants/backend/python/util_rules/faas.py#L400-L403

• This mostly applies to GCF, where we don't have any built-in complete_platforms: https://github.com/pantsbuild/pants/blob/2e5c50511f809aa3c007bb54924277c3b0348f64/build-support/bin/generate_faas_complete_platforms.py#L19-L22
• It also applies to AWS Lambda, for things outside the known set (e.g. Python 3.12): https://github.com/pantsbuild/pants/blob/2e5c50511f809aa3c007bb54924277c3b0348f64/src/python/pants/backend/awslambda/python/target_types.py#L106-L113

(https://github.com/pantsbuild/pants/issues/20515 is potentially related, although not yet confirmed.)