[Typography] Remove deprecated Typography variants and v4 changes

[joshwooding]

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

• Remove deprecated variants

• Rename headlineMapping to variantMapping

• Add Typography display property

Breaking changes

• Remove the deprecated typography variants. You can upgrade by performing the following replacements:
  • display4 => h1
  • display3 => h2
  • display2 => h3
  • display1 => h4
  • headline => h5
  • title => h6
  • subheading => subtitle1
  • body2 => body1
  • body1 (default) => body2 (default)
• Remove the opinionated display: block default typograpghy style.
  You can use the new display?: 'initial' | 'inline' | 'block'; property.
• Rename the headlineMapping property to better align with its purpose.

-<MuiTypography headlineMapping={headlineMapping}>
+<MuiTypography variantMapping={variantMapping}>

Closes #12741

[joshwooding]

should this be default instead?

[oliviertassinari]

Initial is a valid CSS value: https://developer.mozilla.org/en-US/docs/Web/CSS/display. I can't find default.

[joshwooding]

I’m going to remove the default colour change from this PR to make it easier to see if things have been affected.

[joshwooding]

The argos changes left are because of the subheading -> subtitle1 change for the menu and the display block change affecting spacing. I've left the display block change one to get some feedback

[eps1lon]

I would prefer if we kept PRs isolated in the future. This does now include:

• a breaking change with a deprecation in place (typo v2)
• a breaking change without a deprecation (headlingMapping)
• a breaking change without a deprecation (no more block for Link)
• feature

Each of those things should be kept in separate PRs in the future. Apart from following "a commit should do one thing" the change in bundle size cannot be linked to any particular change now and review is harder. For example it wasn't obvious why we removed block in Link at first until I had read the PR summary again.