WL#8393: Increase max-allowed-packet compiled default

Affects: Server-8.0   —   Status: Complete   —   Priority: Medium

Default value of variable 'max_allowed_packet' at server start should be :
67108864 (64M)

** Advantages **
- Less users receive errors about insert or query being larger than 
max_allowed_packet.
- Opportunity to harmonize defaults with clients that explicitly set a larger 
value than the default is anyway (mysqldump always sets 20M even though the server 
defaults to 4M)
Default value of variable 'max_allowed_packet' at server start should be :
67108864 (64M)
Change the default value for max-allowed-packet variable.
   Old default : 4194304  (4M)
   New default : 67108864 (64M)

Doc Link :
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_max_allowed_packet