[APIM][Migration]Update the document with latest migration clients

[Mariangela]

Description:

Chaminda Jayawardena reported as follows: Please update the document [1] with valid/bug-fixed versions of migration clients/references.

Currently, it has linked to an old jar[2] which misses several necessary fixes. Better review all the sections and update wherever necessary.

[1] https://docs.wso2.com/display/AM260/Upgrading+from+the+Previous+Release#200 [2]https://docs.wso2.com/download/attachments/56986329/wso2is-5.x.x-migration.zip?version=13&modificationDate=1555049974000&api=v2

[Mariangela]

Updated the following pages with the link to the latest IS migration client (https://github.com/wso2-extensions/identity-migration-resources/releases/latest)

• https://docs.wso2.com/display/AM260/Upgrading+from+the+Previous+Release

• https://docs.wso2.com/display/AM250/Upgrading+from+the+Previous+Release+in+a+Distributed+Deployment

• https://docs.wso2.com/display/AM250/Upgrading++from+the+Previous+Release+for+a+Single+Node+Deployment

• https://docs.wso2.com/display/AM220/Upgrading+from+the+Previous+Release

[Mariangela]

Updated the APIm 3.0.0 docs with this PR https://github.com/wso2/docs-apim/pull/635