Finish migration from maxInterStageShaderComponents to maxInterStageShaderVariables

[ErichDonGubler]

Connections

• Fixes part of Update inter-stage interface validation to follow spec #5577.
• maxInterStageShaderVariables was originally discussed for introduction to the WebGPU spec. committee at Should we constrain the location of user input-output stage variables WGSL gpuweb/gpuweb#1962.
• maxInterStageShaderVariables was proposed to outright supersede maxInterStageShaderComponents with maxInterStageShaderComponents seems to overlap with maxInterStageShaderVariables gpuweb/gpuweb#4688.
• See also Add note to WebGPU spec explaining what limits the number of WGSL @locations. gpuweb/gpuweb#4077.

Description

TODO

Testing

TBD.

Squash or Rebase?

TBD.

Checklist

• If this contains user-facing changes, add a CHANGELOG.md entry.