[dev] Horde_Cache deprecations

[Ralf Lang]

Hello,

I am currently adapting Horde_Cache with my usual package

- namespaces, PHP 8 related fixes
- PSR-4
- native param and return types, where applicable
- PHPUnit 9.x
- phpstan and phpunit CI jobs
- remove underscore prefixes

In this process I am removing some backends.
- APC becomes APCu as nobody runs APC on the intended PHP 7.4+ baseline
- Eaccelerator backend is removed, as Eaccelerator is dead
- XCache is dead, too

In case anybody has objections or comments, please speak up.

Regards

Ralf

-- 
Ralf Lang
Linux Consultant / Developer
Tel.: +49-170-6381563
Mail: lang at b1-systems.de
B1 Systems GmbH
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