Add Release Drafter configuration

[vgaidarji]

What has been done

1. Added Release Drafter integration.
   Read more at https://github.com/jenkinsci/.github/blob/master/.github/release-drafter.adoc and https://jenkins.io/blog/2019/10/21/plugin-docs-on-github/.
2. Configured GitHub labels to be used for Pull Requests for automatic release generation by Release Drafter
   
3. Manually created releases for versions published after old Changelog deprecation under https://github.com/jenkinsci/plot-plugin/releases. Followed the same Release Drafter format.

TODO

• [ ] Backup old Changelog into current repository as markdown file
  Will be addressed in separate PR.

How to test

1. Will be tested during next release. Release Drafter should create draft release and automatically include labeled PRs

Checklist

• Git commits follow best practices
• Build passes in Jenkins
• Appropriate tests or explanation to why this change has no tests
• JIRA issue is well described (problem explanation, steps to reproduce, screenshots)
• For dependency updates: links to external changelogs and, if possible, full diffs

[vgaidarji]

This plugin still uses full semver and not major.minor as other Jenkins plugins.

[codecov]

Codecov Report

Merging #60 into master will not change coverage by %.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #60   +/-   ##
=========================================
  Coverage     43.38%   43.38%           
+ Complexity      191      190    -1     
=========================================
  Files            18       18           
  Lines          1263     1263           
  Branches        192      192           
=========================================
  Hits            548      548           
  Misses          651      651           
  Partials         64       64           

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