Improve admin mapping change output

[jwils]

Summary

• Move mapping-removal noise handling into elasticgraph-admin so all admin users benefit.
• Preserve current mapping fields in index update payloads and diffs--for concrete indices and index templates--hiding unsupported/unsafe field removals while still showing new fields and mapping parameter changes.
• Keep settings diffs complete, and report new index templates as created instead of dumping the full config diff.

This is purely an output improvement: no behavior changes. Index templates keep preserving fields that have been removed from the schema (as they always have); the removals just no longer clutter every admin run's diff. Actually dropping stale template fields safely is implemented separately in the stacked PR #1303, per review feedback.

Context

Mapping removals are unsupported by Elasticsearch/OpenSearch concrete indices, so showing removed fields every admin run just makes the diffs harder to read. Index templates technically allow field removal, but dropping a field that an older deployed indexer can still write causes indexing failures on new rollover indices (mappings are dynamic: strict), so templates keep their existing fields and the removals are likewise hidden from the diff output.

This addresses #766 and the review feedback to improve elasticgraph-admin directly instead of adding an elasticgraph-admin_lambda opt-in.

Verification

• script/lint
• script/spellcheck
• script/type_check
• script/quick_build

[ellisandrews-toast]

The implementation looks good to me. Only one small comment about whether to enhance user-facing output so the riskiest part is at least explicitly called out.