Migrate deprecated symfony AbstractCache to AbstractAdapter

[emteknetnz]

A couple of community members noted that colymba/gridfield-bulk-editing-tools 3.0.0 was throwing (unrelated) symfony deprecation warnings with framework 4.7.3 that was breaking the bulk upload field.

I replicated this locally with SS_ENVIRONMENT_TYPE="live"

Deprecated: The "Symfony\Component\Cache\Simple\AbstractCache" class is deprecated since Symfony 4.3, use "Symfony\Component\Cache\Adapter\AbstractAdapter" and type-hint for "Symfony\Contracts\Cache\CacheInterface" instead. in /var/www/vendor/symfony/cache/Simple/AbstractCache.php on line 23

• FileSystemCache: Deprecated since Symfony 4.3, use FilesystemAdapter and type-hint for CacheInterface instead. #9955
• Uncaught syntax error when uploading more than 2 images silverstripe-gridfield-bulk-editing-tools#214

The reason there was that colymba/gridfield-bulk-editing-tools was making so much of a fuss about deprecated classes that it caused itself to break

ACs

• migrate the symfony AbstractCache to AbstractAdapter for the rest of framework

[dhensby]

Can this be closed now?

[emteknetnz]

Nope. We still need to migrate off the deprecated Symfony class at some point.