Add CurvedAnimation migration guide

[adil192]

Description of what this PR is changing or adding, and why:

• Adds a migration guide for the breaking change/deprecation from [Breaking Change] Rename CurvedAnimation to ReversibleCurvedAnimation for disambiguation flutter#185501.

Issues fixed by this PR (if any):

• None

PRs or commits this PR depends on (if any):

• [Breaking Change] Rename CurvedAnimation to ReversibleCurvedAnimation for disambiguation flutter#185501

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