docs: Update "Upgrading to v2" guide #370

Job	Run time
Type check / Type check (3.11)	13s
Lint check / Lint check (3.10)	8s
Lint check / Lint check (3.11)	8s
Lint check / Lint check (3.13)	8s
Type check / Type check (3.13)	15s
Type check / Type check (3.10)	14s
Type check / Type check (3.12)	17s
Lint check / Lint check (3.12)	12s
Async dostrings check / Async docstrings check	15s
Unit tests / Unit tests (ubuntu-latest, 3.12)	26s
Unit tests / Unit tests (ubuntu-latest, 3.10)	25s
Unit tests / Unit tests (ubuntu-latest, 3.11)	25s
Docs check / Docs check	1m 3s
Unit tests / Unit tests (ubuntu-latest, 3.13)	24s
Unit tests / Unit tests (windows-latest, 3.13)	36s
Unit tests / Unit tests (windows-latest, 3.10)	40s
Unit tests / Unit tests (windows-latest, 3.12)	38s
Unit tests / Unit tests (windows-latest, 3.11)	36s
Integration tests / Approve integration tests for PRs from forks	8s
Integration tests / Integration tests	0s
Integration tests / Integration tests for PRs from forks	0s
	7m 11s

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