chore(xml): standardize XML declaration encoding to lowercase utf-8 (#1336)

[amasen02]

Summary

Standardizes the XML declaration encoding casing to lowercase encoding="utf-8" across the codebase, as proposed and discussed in #1336.

Changes

• Updated the remaining 21 XML test files under src/Lucene.Net.Tests.QueryParser/Xml/ and src/java/index-compat/pom.xml from encoding="UTF-8" to lowercase encoding="utf-8".
• Aligns all XML declaration headers across the repository with modern conventions and existing standards in the project.

Closes #1336.

[amasen02]

Following up on the XML declaration encoding standardization. Ready to split or reformat the changeset if the diff is too broad.

[paulirwin]

Thanks for the PR!