Trim the migration guide to genuine v1-to-v2 breaking changes #3672

Job	Run time
checks / pre-commit	53s
checks / docs	41s
checks / readme-snippets	13s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 24s
checks / test (3.10, locked, windows-latest)	1m 46s
checks / test (3.11, locked, windows-latest)	1m 42s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 25s
checks / test (3.11, lowest-direct, windows-latest)	1m 39s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.10, lowest-direct, windows-latest)	1m 56s
checks / test (3.14, lowest-direct, windows-latest)	1m 49s
checks / test (3.12, locked, windows-latest)	1m 54s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 18s
checks / test (3.13, locked, ubuntu-latest)	1m 14s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.13, locked, windows-latest)	1m 34s
checks / test (3.11, locked, ubuntu-latest)	1m 5s
checks / test (3.12, lowest-direct, windows-latest)	1m 57s
checks / test (3.14, locked, windows-latest)	1m 57s
checks / test (3.10, locked, ubuntu-latest)	1m 10s
checks / test (3.14, locked, ubuntu-latest)	1m 23s
checks / test (3.12, locked, ubuntu-latest)	1m 34s
checks / test (3.13, lowest-direct, windows-latest)	2m 15s
all-green	3s
	33m 30s

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