fix: stop requiring the drifted redirect-producer fork

[JakeSCahill]

Summary

• Removes the require: '@redpanda-data/docs-extensions-and-macros/extensions/modify-redirects' line from this repo's Antora playbook.

Why

docs-extensions-and-macros#273 deletes the hand-rolled produce-redirects.js fork and its ./extensions/modify-redirects export subpath — Antora's own site-generator already calls the real @antora/redirect-producer unconditionally, so the fork was redundant (and had drifted a real bug: it didn't unpublish self-referential aliases the way upstream does).

Once that version is published, any build that still requires the deleted subpath fails immediately with ERR_PACKAGE_PATH_NOT_EXPORTED, before rendering a single page. This was found by a real Antora build against the full playbook, not just docs-extensions-and-macros' own unit tests (which don't exercise any downstream playbook).

Confirmed via gh api that this line is still present in six repos today; docs (#1960), cloud-docs (#689), and redpanda-labs (#295) already have this exact fix open. This PR covers this repo.

Test plan

• Verify the Antora build still succeeds after this line is removed (no facility change needed — the built-in redirect-producer runs automatically either way).
• Merge alongside or after docs-extensions-and-macros#273, not before it also lands.