WL#10958: Deprecate group_replication_primary_member status variable

EXECUTIVE SUMMARY
=================

This worklog adds a deprecation warning to the documentation regarding
the global status variable group_replication_primary_member. This has
been superseded by the column MEMBER_ROLE in
performance_schema.replication_group_members.

NOTE: There is no framework in the server to deprecate specific global
status variables, thence as it has happened before in similar situation
we are deprecating in the docs only.

Background
==========

On MySQL 5.7, group_replication_primary_member status variable it used
to identify the primary member on a group deployed on single-primary
mode, more precisely, it shows the UUID of the primary member in the
current view.

On MySQL 8.0.2, this information can be extracted from
performance_schema.replication_group_members table, more precisely the
column MEMBER_ROLE will identify each member has PRIMARY or SECONDARY.
MEMBER_ID column it is the member UUID.

+----------------+----------+------+-----+---------+-------+
| Field          | Type     | Null | Key | Default | Extra |
+----------------+----------+------+-----+---------+-------+
| CHANNEL_NAME   | char(64) | NO   |     | NULL    |       |   
| MEMBER_ID      | char(36) | NO   |     | NULL    |       |   
| MEMBER_HOST    | char(60) | NO   |     | NULL    |       |   
| MEMBER_PORT    | int(11)  | YES  |     | NULL    |       |   
| MEMBER_STATE   | char(64) | NO   |     | NULL    |       |   
| MEMBER_ROLE    | char(64) | NO   |     | NULL    |       |   
| MEMBER_VERSION | char(64) | NO   |     | NULL    |       |   
+----------------+----------+------+-----+---------+-------+
https://dev.mysql.com/doc/refman/8.0/en/replication-group-members-table.html

This makes group_replication_primary_member status variable unneeded,
so it should be deprecated in 8.0 and removed in a later version.

User Visible Changes
====================

None. All the changes are done in the manual.

Upgrades
========

After using MySQL 8, the user will be advised to move to use a different
infrastructure to discover and monitor primary member fluctuations. This
however, shall not break backwards compatibility.

Security
========

There are no security considerations.

Observability
=============

There are no observability considerations. There is no new
functionality added or removed.

LOW LEVEL CHANGES
=================

None.