Breaking change

• The static variant has been merged into the determinate variant, with the latter assuming the appearance of the former.
  The removed variant was rarely useful. It was an exception to Material Design, and was removed from the specification.

  -<CircularProgress variant="determinate" />

  -<CircularProgress variant="static" classes={{ static: 'className' }} />
  +<CircularProgress variant="determinate" classes={{ determinate: 'className' }} />

NB: If you had previously customized determinate, your customizations are probably no longer valid. Please remove them.

• I have followed (at least) the PR section of the contributing guide.

Motivation: #7223 (comment)
Closes #20548