docs: update migration guide

[tatomyr]

What/Why/How?

Updated migration guide following #2172.

Security

• The security impact of the change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: e3035d2

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[github-actions]

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	928.4 ± 16.1	896.1	948.8	1.00
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	1160.5 ± 16.2	1129.7	1183.4	1.25 ± 0.03