Pin connection_pool to ~> 2.5 to fix boot failure

[JuanVqz]

Problem

Deploy failed during `rake assets:precompile`:

```
ArgumentError: wrong number of arguments (given 1, expected 0)
connection_pool-3.0.2 ConnectionPool#initialize
```

`connection_pool 3.0.2` changed `initialize` to keyword-only arguments, removing the positional hash form. `activesupport 8.0.3` still calls `ConnectionPool.new(pool_options)` with a positional hash, which breaks on boot.

This was introduced by `bundle update sentry-ruby sentry-rails sentry-sidekiq` which transitively bumped `connection_pool` from `2.5.4` to `3.0.2`.

Fix

Re-ran `bundle update sentry-ruby sentry-rails sentry-sidekiq --conservative` to keep `connection_pool` at `2.5.5` (closest to original `2.5.4`) without adding an explicit Gemfile entry. The lockfile constraint is sufficient — no direct dependency needed.

Boot verified locally. `Gemfile.next.lock` was already at `2.5.4` and required no changes.

Note: `connection_pool 3.0.2` release notes mention a backwards-compat fix for `:name` keyword (issue #210) but that is unrelated — the breaking change is the broader positional→keyword-only `initialize` API change introduced in 3.0.