Updated PR , issue template and slack channel

[himanshu-kun]

What this PR does / why we need it:

• updates PR and issue template
• updates .ci/pipeline_definitions to enable gardener robot to post release notifications on gardener-dwd channel
  Which issue(s) this PR fixes:
  Fixes [Feature] Configure slack for release notifications #74

Special notes for your reviewer:

Release note:

* More categories are added to label a release note for a PR on DWD.
* Release notifications would now be sent to `gardener-dwd` channel (private) on releases.

[gardener-robot-ci-1]

This PR proposes changes that would break the pipeline definition:

<invalid YAML>: while parsing a block mapping
  in "<unicode string>", line 20, column 12:
               inputs:
               ^
expected <block end>, but found '<block mapping start>'
  in "<unicode string>", line 25, column 13:
                registry: 'gcr-readwrite'
                ^

[gardener-robot-ci-3]

The pipeline-definition has been fixed.