Bumped ONNX to 1.21

[klemen1999]

Purpose

• Bumped ONNX to 1.21.0 in requirements.txt.
• Added onnx_compatibility.py to isolate ONNX 1.21 compatibility helpers.
• Patched missing ONNX helper conversions before importing GraphSurgeon so onnx_graphsurgeon continues to work with ONNX 1.21.
• Centralized ONNX model saving, including external-data handling, to avoid repeated save logic and sidecar overwrite issues.
• Made ONNX simplification fallback gracefully when onnxsim fails instead of aborting export.
• Tightened ONNX shape parsing so symbolic or missing dimensions are handled explicitly instead of silently reading as 0.
• Extended ONNX dtype support only for the additional types needed by current code paths: BFLOAT16, INT4, and UINT4.
• Added focused compatibility regression coverage in test_onnx_compatibility.py.

Specification

None / not applicable

Dependencies & Potential Impact

None / not applicable

Deployment Plan

None / not applicable

Testing & Validation

None / not applicable

[klemen1999]

This PR is mostly introduced because of vulnerabilities in ONNX<=1.20.0. We still have to use lower ONNX versions in some parts (RVC4 and Hailo) but we don't have control over that.

[kozlov721]

LGTM