[CATALYST-1815] Add Catalyst 1.6.0 release notes

[jorgemoya]

CATALYST-1815

What changed?

• Adds release notes for Catalyst v1.6.0
• Documents reCAPTCHA v2 support added to the registration, contact us, and product review forms
• Includes a migration guide for threading reCAPTCHA dependencies, server-side validation, and component prop changes

Release notes draft

• Catalyst v1.6.0 adds reCAPTCHA v2 support to the registration, contact us, and product review forms. When reCAPTCHA is enabled in the BigCommerce admin, a verification widget will automatically appear on these forms, helping protect your storefront from spam and bots.

Anything else?

• Refers to the Catalyst 1.6 changeset for full diff reference
• Translation updates were omitted from the release notes as a minor patch-level detail

ping @bigcommerce/team-trac

[rtalvarez]

lgtm