[MCHANGES-467] FeedGenerator.export does not know encoding

**[Elliotte Rusty Harold](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=elharo)** opened **[MCHANGES-467](https://issues.apache.org/jira/browse/MCHANGES-467?redirect=false)** and commented

The use of a writer instead of an output stream in FeedGenerator,.export means that we don't know the encoding and can't set it in the feed, which we should.

Deprecate this method and replace it with one that takes an OutputStream and sets the encoding to an explicit UTF-8 which in 2025 is the only one we should use anyway.

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**Remote Links:**
- [GitHub Pull Request #118
](https://github.com/apache/maven-changes-plugin/pull/118)

Start at FeedGenerator.export and review the existing writer-based API and the proposed OutputStream replacement. Check GitHub Pull Request #118 for the work already associated with this issue. Done means the old method is deprecated, the replacement accepts an OutputStream, and the feed declares explicit UTF-8 encoding.

Written by the indexing model from the issue text.