Rename 'nightly' functions to 'prebuilt' in release scripts (#56753)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56753

The functions getLatestHermesNightlyVersion() and updateHermesVersionsToNightly() were
misleadingly named: they actually fetch from the 'latest-v1' npm dist-tag, not a 'nightly'
tag. Rename them to better reflect what they do:
- getLatestHermesNightlyVersion -> getLatestHermesVersion
- updateHermesVersionsToNightly -> updateHermesVersionsToPrebuilt

Update all call sites in publish-npm.js and publish-npm-test.js.

## Changelog:
[Internal] -

Reviewed By: cortinico

Differential Revision: D104649629

fbshipit-source-id: b1d60b9bb48ef51946e99877a73d5760db585a4c