[TASK] Remove _DEFAULT_PI_VARS from TypoScript

With https://review.typo3.org/c/Packages/TYPO3.CMS/+/75655 the `AbstractPlugin` was made internal.

Option `_DEFAULT_PI_VARS` was only used in the `AbstractPlugin` and can therefore be removed from the TypoScript API.

Additionally `_LOCAL_LANG.[lang-key].[label-key]` now only works for Extbase plugins, the description needs to be adjusted.

Releases: main
Resolves: #98369
Change-Id: I2cf71542f02b5637e8bcf782843e7baa0cb54acd
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75772


Reviewed-by: Chris Müller <typo3@krue.ml>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Lina Wolf <112@linawolf.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Lina Wolf <112@linawolf.de>