TYPO3 deprecations

[b3nkai]

Hi,

There's some deprecations which should be treated. I didn't check all code but it should be done. Running a rector should also be a good start :)

Here's what I noticed:

• The "TYPO3_MODE" constant should be replaced by "TYPO3" since TYPO3 11

https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/11.0/Deprecation-92947-DeprecateTYPO3_MODEAndTYPO3_REQUESTTYPEConstants.html

• The array $GLOBALS['TYPO3_CONF_VARS']['EXT']['extConf'] should be replaced by $GLOBALS['TYPO3_CONF_VARS']['EXTENSIONS'] since TYPO3 9

https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/9.0/Deprecation-82254-DeprecateGLOBALSTYPO3_CONF_VARSEXTextConf.html

[pniederlag]

@abeutel patch with fix for extConf is submitted as PR

I'd suggest to stick with TYPO3_MODE yet still for a while as it widens the range of supported TYPO3 versions without effort for maintainers