docs(instrumentation-fastify): deprecate instrumentation in favor of @fastify/otel

[pichlermarc]

Which problem is this PR solving?

Ownership of the instrumentation is moving to https://github.com/fastify/otel. Fastify will provide and maintain an official instrumentation (@fastify/otel) going forward.

See #2647

Short description of the changes

This PR deprecates @opentelemetry/instrumentation-fastify (once merged, I will follow up with deprecation on npm)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.79%. Comparing base (e57f3e4) to head (c141c52).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2651   +/-   ##
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  Coverage   90.79%   90.79%           
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  Files         169      169           
  Lines        8061     8061           
  Branches     1646     1646           
=======================================
  Hits         7319     7319           
  Misses        742      742           

Files with missing lines	Coverage Δ
...try-instrumentation-fastify/src/instrumentation.ts	95.38% <ø> (ø)