chore(deps): update next and @opennextjs/cloudflare in one Dependabot group

[anurag629]

What and why

Dependabot opens one PR for next and another for @opennextjs/cloudflare. That's how v0.7.0 broke: #169 moved next to 16.4.0 while OpenNext stayed on 1.20.10, which can't run Next 16.4, and every page threw until #184 moved both.

This adds a next group with next, @opennextjs/cloudflare and eslint-config-next, so their updates arrive in one PR and get tested together. Dependabot puts a dependency in the first group it matches, so the group sits above dev-dependencies; otherwise eslint-config-next would stay in that one.

What it doesn't cover:

• Groups apply to version updates only (the default for applies-to), so a security update for next still comes on its own.
• If Next ships before OpenNext supports it, the grouped PR still has the new next with the current OpenNext.

#186 is what catches both of those: CI now runs the built Worker, so a bad pair fails the PR instead of production.

How I tested this

• Parsed the file with PyYAML: next has the three patterns and comes before dev-dependencies.
• Checked the keys and the first-match rule against GitHub's Dependabot options reference.

The first real test is the next weekly run.

Type of change

• Refactor or cleanup