[system] Replace enableSystem with defaultMode

[siriwatknp]

BREAKING CHANGE

enableSystem has been replaced with defaultMode to make it possible to force the dark mode initially.

getInitColorSchemeScript({
-  enableSystem: true,
+  defaultMode: 'system',
})

---

Found the issue while writing the doc in #33958.

Currently, it is not possible to initially force dark mode. The fix is to follow the props in CssVarsProvider by replacing enableSystem with defaultMode.

// developer can pick the default mode that they want

// This means the initial color scheme will be the same as user's system preference
getInitColorSchemeScript({ defaultMode: 'system' })

// This means the initial color scheme will be the dark color scheme
getInitColorSchemeScript({ defaultMode: 'dark' })

---

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

Details of bundle changes

Generated by 🚫 dangerJS against c0b607b

[mnajdova]

The logic looks good, please add a BREAKING CHANGE description and label before merging.