[NEOS-1688] account_id is not needed on ValidateJobMappingsRequest

[nickzelei]

it can be inferred from the connection_id. Should be deprecated. and removed in v0.6.

In the mean time, we should mark it as deprecated in the protos and stop passing it around.

The method already calls connectionservice.GetConnection which ensures the calling user is in the connection's account.

We can deprecate the field by adding a comment to the top @deprecated - <explain> as well as adding a [deprecated = true] to the value itself

string account_id = 1 [deprecated = true];

_{From SyncLinear.com | NEOS-1688}