Use Dockerfile LABEL construct for defining the maintainer

[chirangaalwis]

Description:
Use the Dockerfile LABEL construct instead of using the deprecated MAINTAINER construct as per recommended Dockerfile best practices [1].

[1]: Using MAINTAINER construct

Affected Product Version:
Docker resources for WSO2 Stream Processing v4.4.x and below

Sub Tasks:

• Improved in Alpine based Docker resources
• Improved in CentOS based Docker resources
• Improved in Ubuntu based Docker resources
• Code/Peer review