[Hacktoberfest] Use the concurrency keyword to limit the concurrency of the github action's workflow run.

[ahmetoz]

Situation
To limit the github action workflow concurrency we are using a github action step from https://github.com/softprops/turnstyle

commercetools-sync-java/.github/workflows/ci.yml

Lines 26 to 32 in 7cacc37

	- uses: actions/checkout@v2
	- name: serializing workflow runs
	uses: softprops/turnstyle@v1
	with:
	same-branch-only: false
	env:
	GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

This is needed because running build in parallel causing data conflicts related to the integration tests.

Complication
While we are building the first action flow, github was not supporting this feature, but now seems github introduced concurrency keyword to limit the concurrency in the action.
See: https://github.blog/changelog/2021-04-19-github-actions-limit-workflow-run-or-job-concurrency/

Resolution
Use the concurrency keyword to limit the concurrency of the workflow run.

Ensure you have changed all workflows in the folder: https://github.com/commercetools/commercetools-sync-java/tree/master/.github/workflows

CONTRIBUTING, CODE_OF_CONDUCT

[ahmetoz]

Issue is resolved.