Change default value of explicit_defaults_for_timestamp to ON.
Add deprecation warning when the value of
explicit_defaults_for_timestamp is changed from TRUE to FALSE.

In MySQL 5.6 explicit_defaults_for_timestamp option was introduced (and
deprecated) in WL#6292 with the intention of suggesting users to switch
to new behavior. In most cases though, users ignore the warnings from
explicit_defaults_for_timestamp. This worklog will change the default
value of explicit_defaults_for_timestamp to TRUE, with users able to
revert back to the FALSE behavior in mysql-8.0.

Fix test cases to execute with default behavior by:
i>   Record new default value of explicit_defaults_for_timestamp option.
ii>  Record the result difference by not promotion of timestamp columns
     by default.
iii> Timestamp columns are now nullable by default and no auto
     promotion happens. Record test output.
iv>  Test tried to insert ZERO date in NO_ZERO_DATE mode. This
     is not allowed for timestamp column by default. Adjusted test case.
v>   Fix sql mode to test the intended behavior of the test and
     record result file.
vi>  There are error scenario to check that CREATE TABLE statement fails
     with 2 timestamp column with NO_ZERO_DATE sql. This is deprecated
     behavior. Removed the test scenario.
vii> Record the extra warning when trying to insert NULL to
     TIMESTAMP NOT NULL column. Value inserted in the table changed
     from current_timestamp value to '0000-00-00 00:00:00'.