Update Jira links in How To Contribute guide

FLINK-30007 added a description in the community docs on how to get access to the Flink Jira. But several other locations mentioned and link Jira as well (especially in the how to contribute sections). Newcomers might be miss the paragraph in community and wonder how they could get a working Jira account.

This issue is about replacing all the Jira links (where it's useful) by a reference to the issue track section in the community docs (https://flink.apache.org/community.html#issue-tracker). That way, they are redirected to the information on how they can gain access to the Flink jira board.

Locations that needs to be updated are (not exclusively!):

• https://flink.apache.org/contributing/contribute-code.html
• https://flink.apache.org/gettinghelp.html
• https://flink.apache.org/contributing/how-to-contribute.html