[CssBaseline] Change body font size to body1 (1rem)

[mbrookes]

Breaking changes

• [CssBaseline] The body font size has changed from theme.typography.body2 (0.875rem) to theme.typography.body1 (1rem).
  To return to the previous size, you can override it in the theme:

  const theme = createMuiTheme({
    components: {
      MuiCssBaseline: {
        styleOverrides: {
          body: {
            fontSize: '0.875rem',
            lineHeight: 1.43,
            letterSpacing: '0.01071em',
          },
        },
      },
    },
  })

  (Note that this will also affect use of the Typography component with the default body1 variant).

---

Closes #17100
Part of #20012

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 53e3f1c

[ed-sparkes]

is this a v5 feature, is it not a bug that should be patched on v4?

[oliviertassinari]

@ed-sparkes it's a bug, but also a breaking change, so it was handled in v5.

[charlielow]

For v4 I worked around this by applying a global override and patching in body2 styles, ex.

const theme = createMuiTheme({
  overrides: {
    MuiCssBaseline: {
      '@global': {
        body: {
          fontSize: '0.875rem',
        },
      },
    },
  },
});