WL#12659: Change group_replication_exit_state_action default option to READ_ONLY

WL#11568: "Group Replication: option to shutdown server when dropping
out of the group" did add the option
group_replication_exit_state_action to MySQL v8.0, which default value
is ABORT_SERVER, that is, when a server instance leaves the group
unintentionally the server shuts itself down.

This option has the values:
  ABORT_SERVER: the server shuts itself down;
  READ_ONLY:    the server switches itself to super read only mode.

The rationale behind the default ABORT_SERVER it is, to fence the failed
server against reads and writes, since all connections will be dropped.
The READ_ONLY alternative only prevents against writes, that is, while
on READ_ONLY clients can still read data, which most likely is outdated.

Please refer below links for other details of system variable
group_replication_exit_state_action:
https://dev.mysql.com/doc/refman/8.0/en/group-replication-
options.html#sysvar_group_replication_exit_state_action
WL#11568: Group Replication: option to shutdown server when dropping out of the
group

USER STORIES
============
After gathering customer/community feedback, we revisited this
decision and decided to go with the READ_ONLY action by default on
MySQL v8.0.

FR1:
The default value for the system variable group_replication_exit_state_action
must be READ_ONLY.

This WL changes the default value for system variable
group_replication_exit_state_action.

USER INTERFACE
==============
The user can specify the exit state action by setting the system variable:
 - name: group_replication_exit_state_action
 - scope: global
 - dynamic: yes
 - type: Enumeration
 - default: READ_ONLY
 - values: { READ_ONLY, ABORT_SERVER }

No additional test case needed. Below existing test cases cover this WL:
A. gr_variables_default_values and
B. gr_exit_state_action_option

NOTES:
Value of this variable is not shared across group.
Value is used locally by member when member leaves the group un-intentionally.
So there is no cross version implications.

For any additional details please refer "High-Level Specification" of WL#11568:
Group Replication: option to shutdown server when dropping out of the group.

System variable 'group_replication_exit_state_action' was introduced in
WL#11568.
'group_replication_exit_state_action' accepts string value 'READ_ONLY' and
'ABORT_SERVER'.
The string value of the variable is mapped to the following enum:
enum enum_exit_state_action {
    EXIT_STATE_ACTION_READ_ONLY = 0,
    EXIT_STATE_ACTION_ABORT
};
The default value of the system variable 'group_replication_exit_state_action'
will be EXIT_STATE_ACTION_READ_ONLY.