Update default PHP version to 8.4

[nightnei]

Related issues

• Fixes STU-1528

How AI was used in this PR

AI updated tests and I reviewed it

Proposed Changes

We are updating the default PHP version to the version used by wpcom.
Pressable uses the latest - 8.5, but I think that it's better to be aligned with wpcom and use 8.4.

I also considered reusing the constant in tests to avoid a lot of changes in the future PHP updates, but here is the answer from Claude code, and I agree, that's why I decided to keep it as is.
Reuse the constant in tests — bumping the default in the future becomes a one-line change, no test churn. But tests become tautological: a test that does phpVersion: DEFAULT_PHP_VERSION then asserts expect(...).toBe(DEFAULT_PHP_VERSION) doesn't actually check that the right version flows through — it'd pass even if the default were broken.
Keep hardcoded (current state) — tests act as a tripwire: bumping the default forces you to look at every place that depends on it, and assertions verify a concrete value end-to-end.
My take: keep it hardcoded for assertions (they should pin concrete values).

Testing Instructions

1. Try to create a site and assert that 8.4 is preselected during creation flow and is indeed selected when a site is created (just checking it in site settings inside Studio is enough).
2. Holistically test Studio to asert that everythign works well with 8.4 (previously we had a lot of issues with 8.5, so theoretically that something can be broken, e.g. pull/push/import/export/some UI notifications/etc)

I tested pull/push/import db and full site/export db and full site/clicked on different buttons - evetrything works well.

[wpmobilebot]

📊 Performance Test Results

Comparing e61ef83 vs trunk

app-size

Metric	trunk	e61ef83	Diff	Change
App Size (Mac)	1283.00 MB	1283.00 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	e61ef83	Diff	Change
load	1928 ms	1838 ms	90 ms	🟢 -4.7%

site-startup

Metric	trunk	e61ef83	Diff	Change
siteCreation	8113 ms	8135 ms	+22 ms	⚪ 0.0%
siteStartup	4291 ms	4958 ms	+667 ms	🔴 15.5%

---

Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)

[bcotrim]

LGTM 👍
I confirm PHP 8.4 is not the default option when creating sites on Studio CLI and Studio app.