ci(dependabot): remove obsolete enable-beta-ecosystems

`enable-beta-ecosystems: true` was added along with the `pre-commit`
ecosystem, which Dependabot only supported as a beta ecosystem at the
time. Dependabot's `pre-commit` support is no longer in beta, and
GitHub's Dependabot options reference now documents
`enable-beta-ecosystems` as "Not currently in use". It also lists
`pre-commit` among the supported ecosystems with no preview or beta
caveat.

So the option does nothing here, but suggests that something in this
configuration still depends on a beta feature. Remove it.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: EliahKagan

.github/dependabot.yml

@@ -1,5 +1,4 @@
 version: 2
-enable-beta-ecosystems: true
 updates:
 - package-ecosystem: github-actions
   directory: "/"