Refactor: libcrmcommon: New PCMK__META_PROMOTED_MAX_LEGACY constant

Commit ee656e3c created PCMK_XA_PROMOTED_MAX_LEGACY under the assumption
(from task T535) that "promoted-max" was an XML attribute. However, it's
actually a meta-attribute, so that was the wrong naming convention to
use.

Accordingly, this can be internal.

Ref T535

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>