Update CONTRIBUTING.md with regards to discussion forums

[Chealer]

Affects: N/A

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The Discuss section in Contributing to the Spring Framework is confusing.

Its first paragraph points to Stack Overflow, which is not designed to discuss Spring development to my knowledge.

Its second and last paragraph treats issue tracking.

Unfortunately, nothing seems to treat actual discussion about Spring development. And the only other document which should treat that that I can find (the Community page) also fails to mention any discussion forum about Spring development.

I tried searching for forums by myself and only found forum.springframework.org, which was shut down and redirected to a blog post about that shutdown, and a few on SourceForge. The Open Discussion forum on SourceForge is marked as deprecated, pointing to forum.springframework.org as a replacement. The only one which might remain relevant would be springframework-developer, except the last mail there was in February, and the last legitimate message goes back to February 2019.

If springframework-developer remains a proper forum for discussing Spring development, please mention that on at least one of the above documents. If not, please list the other options.

[bclozel]

As you've found out yourself, Spring community forums and mailing lists had many flaws and we've seen significant progress with the community since we've retired those forums and focused on StackOverflow.

The springframework-developer mailing list is not monitored by the Spring team. There are many Java and Spring community forums out there, but these days the Spring team provides free community support on GitHub issue trackers (for bugs and enhancement requests) and StackOverflow (for questions).

We might expand this arrangement in the future, but not for now.

I'll improve the CONTRIBUTING document to better reflect that. Thanks!