Add links to OpenCensus spans after span start

[thswlsqls]

Fixes #6589

Description

• OpenTelemetrySpanImpl.addLink(Link) logged a warning and dropped the link. It now converts the
  OpenCensus link and forwards it to the OpenTelemetry span.
• The warning ("OpenTelemetry does not support links added after a span is created") predates
  Span#addLink, added in 1.37.0, so its premise no longer holds.
• Siblings addAnnotation and addMessageEvent already delegate; addLink was the only one that
  did not.
• Calls getDelegate().addLink(..), not DelegatingSpan.super.addLink(..), which would resolve to
  the no-op default method on Span.
• Link.getType() is dropped — no OpenTelemetry equivalent, and OpenTelemetrySpanBuilderImpl
  ignores link type too.
• Removes the stale "Known Problems" README section; the dropped links bullet was its only entry.

Testing done

• Added OpenTelemetrySpanImplTest#addLink_delegatesToOtelSpan and #addLink_mapsAttributes,
  asserting trace ID, span ID and attributes reach Span#addLink(SpanContext, Attributes). Both
  fail without the fix.
• ./gradlew :opencensus-shim:check — 63 tests passed.
• No public API change, so no apidiff update; added a ### Shims CHANGELOG entry.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.65%. Comparing base (09d6c17) to head (e6f64e9).

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #8635      +/-   ##
============================================
+ Coverage     91.64%   91.65%   +0.01%     
- Complexity    10348    10349       +1     
============================================
  Files          1013     1013              
  Lines         27380    27389       +9     
  Branches       3218     3218              
============================================
+ Hits          25092    25103      +11     
+ Misses         1558     1556       -2     
  Partials        730      730              

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