LPS-44751 Missing logic: Add keys.add(key)

The current issue was caused by SF. 

I tested the previous fix and it can't work successfully due to keys still empty and we didn't add the vaule for keys.

com.liferay.portal.upgrade.v6_2_0.UpgradeSocial.getUniqueModifiedDate() method
##
if (keys.contains(key)) { // the if block will be never executed because keys still empty.
continue;
}
##

The original issue was caused by existing the same data in these fields "groupId, companyId, userId, modifiedDate,resourcePrimKey" of different wikipage versions. n 5.2.8 version, we created "unique index IX_8F32DEC9 on SocialActivity (groupId, userId, createDate, classNameId, classPK, type_, receiverUserId);" When upgrade to 6.2, due to existed the duplicated data indexed in "unique index IX_8F32DEC9" so that the issue occured. 

As for the detailed explanation regarding the original issue, please refer to the below link:

https://issues.liferay.com/browse/LPP-10304?focusedCommentId=423575&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-423575


As for the current fix logic, in the process of upgrading, I think we modified modifiedDate for duplicate modifiedDate of different wikipage versions.

Please help check it.