Fix the behavior of CaffeineCache.computeIfAbsent on branch_8x

Spin-off from SOLR-13817, spotted by Andy Webb.

The back-port of CaffeineCache to branch_8x is missing a conditional statement in computeIfAbsent which breaks the contract of this method, i.e. if the loading function returns null the cache mapping should remain unchanged and a null value should be returned.

Note: this only affects branch_8x, master already has this conditional.

This issue also modifies comments in the default solrconfig.xml to indicate that the old cache implementations existing in branch_8x are deprecated in favor of CaffeineCache.