Do not migrate the old typography support if core already did it

[oandregal]

Related WordPress/wordpress-develop#1338

We made a change to how the typography block supports are declared, from:

supports: {
  "fontSize": true,
  "lineHeight": true
}

to

supports: {
  "typography": {
    "fontSize": true,
    "lineHeight": true
  }
}

for which we introduced a migration mechanism. This is being ported to WordPress 5.8, so we don't need to run it in the plugin if core already did it.