Deprecate all `\Redmine\Api\Issue::PRIO_*` constants

[Artur Weigandt]

In Redmine\Api\Issue we have the following constants defined:

• PRIO_LOW = 1
• PRIO_NORMAL = 2
• PRIO_HIGH = 3
• PRIO_URGENT = 4
• PRIO_IMMEDIATE = 5

The IDs of the issue priorities are not hard coded in Redmine and are different betweent every server and could even differ between two projects on the same server.

We should deprecate this constants and remove them in v3.