Update japicmp baseline docs to match pinned apidiffBaselineVersion

[thswlsqls]

No issue: docs incomplete-fix follow-up to #8591, PR-only (precedent: #8491)

Description

• Pin japicmp baseline in version.gradle.kts #8591 pinned the japicmp baseline to apidiffBaselineVersion in version.gradle.kts and removed the latest.release lookup, but api-design.md and gradle-conventions.md still describe the old dynamic behavior.
• Pin japicmp baseline in version.gradle.kts #8591 updated build.md, which delegates with "See api-design.md#japicmp for details" — the accurate doc points readers at a stale one.
• Both docs now name the pinned apidiffBaselineVersion release, mirroring build.md.
• Left alone on purpose: the docs/apidiffs/current_vs_latest/ directory name and the latest path literal in otel.japicmp-conventions.gradle.kts name the output path, not the comparison baseline; Pin japicmp baseline in version.gradle.kts #8591 kept them in build.md too.

Testing done

• Docs-only: no behavior, signature, or public API change, so no tests, no Gradle run, and no apidiff or CHANGELOG entry.
• Verified against otel.japicmp-conventions.gradle.kts lines 20/108 (baselineVersion = apiBaseVersion ?: apidiffBaselineVersion) and version.gradle.kts line 2 (= "1.64.0").
• Grepped docs/knowledge/, buildSrc/, .github/, version.gradle.kts for the old wording — nothing stale remains.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (63ebd94) to head (578cf07).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8616      +/-   ##
============================================
- Coverage     91.63%   91.61%   -0.02%     
- Complexity    10326    10342      +16     
============================================
  Files          1013     1013              
  Lines         27295    27352      +57     
  Branches       3205     3215      +10     
============================================
+ Hits          25011    25058      +47     
- Misses         1558     1566       +8     
- Partials        726      728       +2     

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