Drop our Playwright `CookieJar` once `playwright-symfony` provides one

[kbond]

src/Browser/Session/Playwright/CookieJar.php proxies ->use(CookieJar $jar) to the real browser, because the client's own jar held a copy and mutating it had no effect.

playwright-php/playwright-symfony#52 moves that upstream: getCookieJar() returns a jar backed by the browser context. Once it lands and we require a version with it, this class and tests/Session/Playwright/CookieJarTest.php can go, about 190 lines, and PlaywrightBrowser no longer needs the cookieJar() override.

Already verified: dropping the override and letting getCookieJar() through keeps can_manipulate_cookies and the remember-me test green.