[TASK] Update Fluid to 2.15

This release contains important pre-patches for the upgrade to
Fluid v4 in the next step. Apart from soft deprecations of code that
will log deprecation level errors with Fluid v4, it backports a
feature from Fluid v4 to Fluid v2 to deliver a smoother migration
path from Fluid v2 to v4 in 3rd party code. The necessary changes
to TYPO3's ViewHelpers will be applied in a follow-up patch.

Similarly to the previous update to 2.14.0, this patch adds RST
files that describe both new features and deprecations
that were introduced in this version.

Noteworthy changes:

* The functionality of `CompileWithContentArgumentAndRenderStatic`
has been integrated into `AbstractViewHelper` and can thus be used
with non-static ViewHelpers from now on.
* `renderStatic()` and both associated traits
`CompileWithContentArgumentAndRenderStatic` and
`CompileWithRenderStatic` are deprecated
* Methods deprecated with the introduction of the
ViewFactoryInterface in #104773 have been deprecated in Fluid
as well
* `true`, `false` and `null` can no longer be used as variable names,
as these will become proper language keywords in Fluid v4

Executed composer commands:

> composer req typo3fluid/fluid
> composer req -d typo3/sysext/adminpanel --no-update typo3fluid/fluid
> composer req -d typo3/sysext/core --no-update typo3fluid/fluid
> composer req -d typo3/sysext/fluid --no-update typo3fluid/fluid
> composer req -d typo3/sysext/redirects --no-update typo3fluid/fluid

Resolves: #104789
Related: #104463
Related: #104773
Releases: main
Change-Id: I32a465e327cf84975c45e86580c54d060d688cd6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85837


Tested-by: Jochen Roth <rothjochen@gmail.com>
Reviewed-by: Simon Praetorius <simon@praetorius.me>
Tested-by: Garvin Hicking <gh@faktor-e.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Jochen Roth <rothjochen@gmail.com>
Tested-by: Simon Praetorius <simon@praetorius.me>
Reviewed-by: Garvin Hicking <gh@faktor-e.de>