Update theme.json
schema migrations
#36154
Merged
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Part of #34349
This PR prepares the code that migrates
theme.json
schemas to add v2 in a subsequent PR, see #36155It clarifies how migrations are supposed to work and it proposes that each migration only updates from the latest to the new one. Before this, new migrations were supposed to be ported to all versions. See #34485 for an example. As demonstrated by the block deprecations #35663 trying to migrate everything to the latest version is not going to scale well.
How to test
Verify the tests pass. It only refactors code.