Add automation to manage issues backlog

[codeboten]

As discussed in the SIG meeting we want to use automation to clean up old issues from the backlog. The automation should:

• filter out issues labeled with "feature-request"
• close inactive issues after 60 days
• add a "backlog" label
• add a warning before closing the issue

[lzchen]

In the future, we should stop using "FIXME" tags in code, and instead create long-lasting issues for them and tag them as "backlog".