Renames pnpEnableExperimentalEsm into pnpEnableEsmLoader

[arcanis]

What's the problem this PR addresses?

Using experimental in the name isn't necessary since the setting is enabled by default (so only a segment of those who use it will see the warning), and would require either a compatibility check or users to edit their configuration once we graduate the feature. Note that It'll still be experimental, as explained in the documentation.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[shuding]

The default is still false here:

berry/packages/gatsby/static/configuration/yarnrc.json

Line 589 in ba821ef

"default": false