Use only one way of specifying routing, version and version_type

[alexksikes]

Today we have a couple of ways of specifying the routing, version, and version
type of a given document request.

Beyond camelCase, we use:

_routing or routing
_version or version
_version_type or version_type

This makes the use of ParseField difficult for these parameters, as the camelCase of say _routing isn't routing. We should probably settle on using only one, or make ParseField handle this case.

Relates to #8988

[jpountz]

or make ParseField handle this case

Please no. :-)

+1 on settling on just one

[clintongormley]

Let's settle on a single form (I don't really care if it is _foo or foo) and deprecate the other form (with depr logging), but only remote it in 6.0.

[mariha]

I was thinking of working on this issues, isn't it already fixed though? #27040 (comment)

[jpountz]

Good catch @mariha it has been addressed indeed.