docs: v5 to v6 migration

MIGRATING_V5-V6.md

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+# Migrating `@salesforce/core` from v5 to v6
+
+v5 contains breaking changes to the classes that extend from ConfigStore/ConfigFile. We'll call these the "config stack" as a shorthand
+
+We've had a series of bugs where files controlled by the config stack could lose data or become corrupted when multiple processes try to write them concurrently.
+
+V6 introducing a file locking mechanism previously only available on the `alias` file and a process of resolving conflicts based on timestamps.
+
+But that comes with breaking changes to to reduce the risk of "getting around" the safeties.
+
+## Breaking changes related to the Config Stack
+
+- AuthInfo.getFields now returns a read-only object. Use AuthInfo.update to change values in the fields.
+- `setContents` method is no longer available in the ConfigFile stack. Use `update` to merge in multiple props, or `set/unset` on a single prop.
+- `write` and `writeSync` method no longer accepts a param. Use other methods (`set`, `unset`, `update) to make modifications, then call write()/writeSync() to do the write.
+- the use of lodash-style `get`/`set` (ex: `set('foo.bar.baz[0]', 3)`) no longer works.
+- You can no longer override the `setMethod` and `getMethod`` when extending classes built on ConfigFile. Technically you could override get/set, but DON'T!
+- Everything related to tokens/tokenConfig is gone
+
+## Unrelated changes that we did because it's a major version
+
+- node18+ only, compiles to es2022
+- move `uniqid`` to a shared function, outside of testSetup
+
+## Previously deprecated items that are now removed
+
+- removed sfdc.isInternalUrl. Use new SfdcUrl(url).isInternalUrl()
+- removed sfdc.findUppercaseKeys. There is no replacement.
+- removed SchemaPrinter. There is no replacement.