docs: fix link to moved changelog docs source

[MikeMcC399]

Additional details

The document Cypress App - Managing the Release Changelog, in the section Release, contains an outdated link to the changelog.mdx file in the cypress-documentation repository, which was reorganized in October 2024.

1. Change the changelog.mdx link to https://github.com/cypress-io/cypress-documentation/blob/main/docs/app/references/changelog.mdx
2. Update the example PR to https://github.com/cypress-io/cypress-documentation/pull/5965

Steps to test

Open the document Release section and click on the link changelog.mdx. Confirm it references an existing file. (Note: GitHub may display the error "This blob took too long to generate".)

How has the user experience changed?

Maintainer (release function) issue only.

PR Tasks

• [na] Have tests been added/updated?
• [na] Has a PR for user-facing changes been opened in cypress-documentation?
• [na] Have API changes been updated in the type definitions?

[cypress-app-bot]

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