SOLR-17957 Remove XLSX Response Writer

[janhoy]

https://issues.apache.org/jira/browse/SOLR-17957

This PR sits on top of #3769 and that one must be merged to main first

[Copilot]

Pull Request Overview

Removes the deprecated XLSX Response Writer from Solr as part of SOLR-17957, cleaning up code, tests, and documentation.

• Drops XLSXResponseWriter implementation and its tests from the extraction module
• Removes dynamic registration of wt=xlsx from SolrCore
• Updates documentation: removes XLSX references from the Query Guide, adds an Upgrade Note, and updates CHANGES.txt

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
solr/modules/extraction/src/java/org/apache/solr/handler/extraction/XLSXResponseWriter.java	Removes the XLSXResponseWriter implementation.
solr/modules/extraction/src/test/org/apache/solr/handler/extraction/TestXLSXResponseWriter.java	Removes unit tests tied to the XLSX writer.
solr/core/src/java/org/apache/solr/core/SolrCore.java	Removes dynamic registration of the xlsx writer from DEFAULT_RESPONSE_WRITERS.
solr/solr-ref-guide/modules/query-guide/pages/response-writers.adoc	Removes XLSX from the documented response writers and deletes the XLSX section.
solr/solr-ref-guide/modules/upgrade-notes/pages/major-changes-in-solr-10.adoc	Adds an Upgrade Note announcing removal and migration guidance.
solr/CHANGES.txt	Adds an entry under Deprecation Removals noting the XLSX writer removal.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}