CI solves all dependencies at published bounds without the network-arbitrary allow-newer stopgap

## Summary

Remove the `--allow-newer=network-arbitrary:base,network-arbitrary:QuickCheck`
stopgap from `ci.yml` once a `network-arbitrary` release lifts its `base` and
QuickCheck caps, so CI once again solves every dependency at its published bounds.

## Motivation

`ci.yml` threads that flag through six steps (lines 64, 78, 89, 93, 101, and
126), documented at `ci.yml:46-51`: `network-arbitrary` caps `base < 4.21`, which
excludes GHC 9.12 and 9.14, and lags QuickCheck. Without the relaxation the
matrix can't build its test suite on the newer compilers.

The scoping is careful — it names one package and two of its dependencies, so the
library's own published bounds stay strictly enforced and the sdist-based build
still tests what a consumer faces. The residual gap is narrow but real: the test
suite builds against a resolution no downstream user would get, so a genuine
incompatibility in `network-arbitrary` on 9.12/9.14 is invisible here.

This is bookkeeping against an upstream release, in the same shape as #142
(aeson/network-uri-json), which closed the same way.

## Scope

- Watch alunduil/network-arbitrary#106 for a release lifting the caps.
- Bump the `network-arbitrary` bound in `collection-json.cabal` if needed.
- Delete the `--allow-newer` flag from all six `ci.yml` steps and the explanatory
comment at `ci.yml:46-51`.

## Acceptance criteria

- [ ] No `allow-newer` remains in `ci.yml`.
- [ ] The full GHC matrix is green with every dependency solved at published bounds.

## Additional context

- Upstream: alunduil/network-arbitrary#106.
- Same pattern as #142.

Start by checking alunduil/network-arbitrary#106, then inspect ci.yml lines 46-51 and the six steps at lines 64, 78, 89, 93, 101, and 126. Check collection-json.cabal for the network-arbitrary bound if the upstream release requires it. Done means no allow-newer remains and the full GHC matrix is green with dependencies resolved at published bounds.

索引モデルが issue の本文から書いたものです。