Move ContentRazorHelperExtensions to more abstract project

[infofromca]

No description provided.

[Piedone]

Can you explain the rationale?

[hishamco]

IMHO we need to move such helpers in OC.xxx.Core - if exists -, otherwise OC.xxx.Abstractions this will avoid referencing modules on external projects if we need a simple IOrchardHelper method

[sebastienros]

Can you update the release notes also in the PR to list it as a breaking change? Though technically we don't expect it to break actual websites (it's still technically possible).

[sebastienros]

Also maybe we should do this #17189 at the same time.

This could be a non-breaking change by having the class defined in two namespaces, with the old one as obsolete.