The environment variable CONF_DIR was previously inconsistently used in
our packaging to customize the location of Elasticsearch configuration
files. The importance of this environment variable has increased
starting in 6.0.0 as it's now used consistently to ensure Elasticsearch
and all secondary scripts (e.g., elasticsearch-keystore) all use the
same configuration. The name CONF_DIR is there for legacy reasons yet
it's too generic. This commit renames CONF_DIR to ES_PATH_CONF.
Relates #26197
e9687622bd