The shards preference on a search request enables specifying a list of
shards to hit, and then a secondary preference (e.g., "_primary") can be
added. Today, the separator between the shards list and the secondary
preference is ';'. Unfortunately, this is also a valid separtor for URL
query parameters. This means that a preference like "_shards:0;_primary"
will be parsed into two URL parameters: "_shards:0" and "_primary". With
the recent change to strict URL parsing, the second parameter will be
rejected, "_primary" is not a valid URL parameter on a search
request. This means that this feature has never worked (unless the ';'
is escaped, but no one does that because our docs do not that, and there
was no indication from Elasticsearch that this did not work). This
commit changes the separator to '|'.
Relates #20786
d01a62908a