[cleanup] PIP-462: Remove Etcd metadata store backend

[merlimat]

Summary

• Remove Etcd metadata store implementation and tests
• Remove jetcd-core-shaded module
• Remove jetcd dependencies from pulsar-metadata and pulsar-broker
• Remove Etcd-related SSL test certificates and OWASP suppressions

Motivation

PIP-462: #25319

The Etcd metadata store backend is not used in production deployments. The jetcd library is poorly maintained and brings in significant transitive dependencies, requiring a dedicated shading module. This increases the Pulsar tarball and Docker image size unnecessarily.

ZooKeeper remains the fully supported default metadata store, and Oxia is the supported alternative for users seeking a different backend.

For users who still need Etcd support, it can be maintained as an external plugin via the pulsar-java-contrib repository.

Documentation

• doc-required

(If you update the PR title, please also update Title above, since they are
used in the release notes.)

• doc-not-needed
  (Your PR doesn't need any doc update, such as a typo fix or an internal
  refactoring.)

• doc-complete
  (If your PR needs doc update, please complete the PR in
  apache/pulsar-site.)

Matching PR in forked repository

N/A

Ready to test

• ready-to-test

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.77%. Comparing base (a27988b) to head (068994e).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...pulsar/metadata/impl/MetadataStoreFactoryImpl.java	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #25380      +/-   ##
============================================
- Coverage     72.81%   72.77%   -0.05%     
+ Complexity    33791    33697      -94     
============================================
  Files          1954     1952       -2     
  Lines        154793   154525     -268     
  Branches      17731    17698      -33     
============================================
- Hits         112716   112450     -266     
+ Misses        33066    33043      -23     
- Partials       9011     9032      +21     

Flag	Coverage Δ
inttests	25.74% <0.00%> (-0.17%)	⬇️
systests	22.59% <0.00%> (+0.01%)	⬆️
unittests	73.74% <0.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...pulsar/metadata/impl/MetadataStoreFactoryImpl.java	76.19% <0.00%> (-4.30%)	⬇️

... and 78 files with indirect coverage changes

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