docs: document FastAPI 0.137 regression and fix in 6.26.2

[alexandra5000]

Summary

• Adds a known-issues entry for the FastAPI 0.137 _IncludedRouter AttributeError regression (issue FastAPI 0.137 regression: Elastic APM Starlette middleware crashes on _IncludedRouter (path missing) #2681), including a pin fastapi<0.137 workaround and resolved version (6.26.2, 2026-06-22)
• Updates the Starlette/FastAPI section in supported technologies to note that FastAPI>=0.137 requires elastic-apm>=6.26.2

Closes elastic/docs-content#7084

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Tool(s) and model(s) used: Claude Opus 4.8

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🔍 Preview links for changed docs

• docs/reference/supported-technologies.md
• docs/release-notes/known-issues.md

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[vishaangelova]

Thanks, LGTM! just a tiny suggestion