fix: add trailing slash to retire_certs_s3_for_user URL pattern

[ttak-apphelix]

Description:

The retire_certs_s3_for_user URL was missing a trailing slash. Tubular's get_api_url() always appends a trailing slash, so the POST request hit .../retire_certs_s3_for_user/ which Django redirected (301) to a GET, returning 404 since no GET handler exists.

Changes
added trailing slash to the URL pattern in certificates/urls.py.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

JIRA ticket:

• https://2u-internal.atlassian.net/browse/BOMS-558

[Copilot]

Pull request overview

This PR fixes a routing mismatch for the certificates retirement per-user endpoint by updating the Django URL pattern to include a trailing slash (to match a client that always appends /), and bumps the package version / changelog accordingly.

Changes:

• Add trailing slash to retire_certs_s3_for_user URL route and update the corresponding test request path.
• Bump package version to 7.7.0.
• Add a 7.7.0 changelog entry (but the current diff also introduces accidental duplicate/stray changelog content that should be removed).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
edx_arch_experiments/certificates/urls.py	Updates API route for the per-user retirement endpoint to include a trailing slash.
edx_arch_experiments/certificates/tests/test_views.py	Updates the test POST URL to match the new trailing-slash route.
edx_arch_experiments/__init__.py	Version bump to 7.7.0.
CHANGELOG.rst	Adds 7.7.0 release notes, but also accidentally adds duplicated legacy entries that should be removed.

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