docs: note allowEncodedSlashes removal in migrate-extensions

[Shaurya2k06]

Adds the missing V3.5 removal note for apiml.service.allowEncodedSlashes in the V1→V2 migration table. Complements #5097 / closes #5098. The replacement property is apiml.security.enableStrictUrlValidation (inverse meaning: set to false to allow encoded slashes).

List the file(s) included in this PR:

• migrate-extensions.md — Notes column for APIML_ALLOW_ENCODED_SLASHES updated with the V3.5 removal/replacement guidance.

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