chore: force h2 version to avoid minimal-versions deadlock

[haphungw]

Fix a gcb-pr-minimal-versions build failure in google-cloud-gax-internal tests caused by h2 version 0.4.2, which surfaces after google_cloud_unstable_tracing guards are removed in #5292.

The Problem
During minimal-versions checks, h2 resolves to 0.4.2, which somehow leads to deadlocks or missing tracing spans in our gRPC tests.

Previous Attempts
I tried switching tests to multi_thread (PR #5308), which masked the issue but was not a good solution.
I tried forcing the version in the root [workspace.dependencies], but Cargo ignored it during the isolated package build for gax-internal.

The Fix
Add h2 = "0.4.13" as a direct dependency in src/gax-internal/Cargo.toml. 0.4.13 is the version resolved in our normal Cargo.lock.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.78%. Comparing base (2e516df) to head (8addd96).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5318      +/-   ##
==========================================
- Coverage   97.79%   97.78%   -0.01%     
==========================================
  Files         220      220              
  Lines       45845    45845              
==========================================
- Hits        44832    44830       -2     
- Misses       1013     1015       +2     

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