Update V2 Journeys product limits for concurrent instances and unique identifiers [DOC-1279]

[pwseg]

Proposed changes

• Updates the Journeys V2 product limits page with recent backend limit changes.
• Specifically: Updated "Instances per profile" description to clarify the 25 concurrent instance limit applies across all journeys
• Added 150 character limit for unique identifier property values
• Clarified the distinction between unique identifier property names (500 chars) and property values (150 chars).

Merge timing

• ASAP once approved

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