Update libddwaf dependency to 1.21.0.0.1

[y9v]

What does this PR do?
It updates libddwaf gem dependency to 1.21.0.0.1

Motivation:
We have to update libddwaf to enable system tests for SQLi and SSRF features.

Change log entry
None. This is internal change.

Additional Notes:
None.

How to test the change?
CI and manual testing

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (ef85993) to head (f55aafb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4537      +/-   ##
==========================================
- Coverage   97.76%   97.75%   -0.01%     
==========================================
  Files        1392     1392              
  Lines       84890    84890              
  Branches     4277     4277              
==========================================
- Hits        82989    82988       -1     
- Misses       1901     1902       +1     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: appsec-update-libddwaf-to-1-21
Commit report: f55aafb
Test service: dd-trace-rb

✅ 0 Failed, 20779 Passed, 1374 Skipped, 3m 12.52s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-03-26 12:02:59

Comparing candidate commit f55aafb in PR branch appsec-update-libddwaf-to-1-21 with baseline commit ef85993 in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 30 metrics, 2 unstable metrics.

scenario:tracing - Propagation - Datadog

• 🟥 throughput [-2723.013op/s; -2656.781op/s] or [-8.636%; -8.426%]

[Strech]

@y9v are you sure that we don't need any mention in the change logs? I think there is some new detectors, or we don't make use of them?

[y9v]

@Strech We are not using any of the new shi_detector and lfi_detector detectors yet. The rest of the changes to libddwaf seem to be internal or related to rules, so I think we can omit the changelog entry here