[agent] Add deprecation notice to the readme

[yurishkuro]

Which problem is this PR solving?

• Part of (deprecation) Remove jaeger-agent #4739

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.43%. Comparing base (f0c5a1d) to head (b61c44b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6130   +/-   ##
=======================================
  Coverage   96.43%   96.43%           
=======================================
  Files         353      353           
  Lines       20135    20135           
=======================================
  Hits        19417    19417           
  Misses        533      533           
  Partials      185      185           

Flag	Coverage Δ
badger_v1	8.32% <ø> (ø)
badger_v2	1.68% <ø> (ø)
cassandra-4.x-v1	14.39% <ø> (ø)
cassandra-4.x-v2	1.62% <ø> (ø)
cassandra-5.x-v1	14.39% <ø> (ø)
cassandra-5.x-v2	1.62% <ø> (ø)
elasticsearch-6.x-v1	18.51% <ø> (ø)
elasticsearch-7.x-v1	18.58% <ø> (ø)
elasticsearch-8.x-v1	18.76% <ø> (ø)
elasticsearch-8.x-v2	1.67% <ø> (ø)
grpc_v1	9.53% <ø> (ø)
grpc_v2	7.00% <ø> (-0.01%)	⬇️
kafka-v1	8.88% <ø> (ø)
kafka-v2	1.68% <ø> (ø)
memory_v2	1.68% <ø> (ø)
opensearch-1.x-v1	18.63% <ø> (-0.01%)	⬇️
opensearch-2.x-v1	18.63% <ø> (-0.01%)	⬇️
opensearch-2.x-v2	1.67% <ø> (ø)
tailsampling-processor	0.47% <ø> (ø)
unittests	95.34% <ø> (ø)

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