feat!: bidirectional streams are GA

[dbolduc]

Fixes #10307

Drop the ExperimentalTag from bidirectional streaming APIs, as discussed in our team meeting.

I decided to just update the ABI dumps. I considered making the following change:

diff --git a/ci/cloudbuild/builds/check-api.sh b/ci/cloudbuild/builds/check-api.sh
index 93ee13e0e..dfd0d9b3e 100755
--- a/ci/cloudbuild/builds/check-api.sh
+++ b/ci/cloudbuild/builds/check-api.sh
@@ -112,8 +112,9 @@ function check_abi() {
       # against the mangled symbol names. For example, 8 is the number of
       # characters in the string "internal", and it should again be followed
       # by some other number indicating the length of the symbol within the
-      # "internal" namespace. See: https://en.wikipedia.org/wiki/Name_mangling
-      -skip-internal-symbols "(8internal|_internal)\d"
+      # "internal" namespace. Skip experimental symbols, denoted by the
+      # `ExperimentalTag`, too. See: https://en.wikipedia.org/wiki/Name_mangling
+      -skip-internal-symbols "(8internal\d|_internal\d|15ExperimentalTag)"
       # The library to compare
       -l "${library}"
       # Compared the saved baseline vs. the dump for the current version

We make the same guarantees about experimental APIs as we do with internal APIs..... but ignoring them in check-api just feels incautious to me. I would be happy to update the build if others disagree, though.

I did not update the CHANGELOG. We can write something for the release.

---

This change is 

[google-cloud-cpp-bot]

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[google-cloud-cpp-bot]

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For commit: bf9ac7a6eed2ff5363a8ee918328acf328249348

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[codecov]

Codecov Report

Base: 93.85% // Head: 93.86% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (bf9ac7a) compared to base (3c03c22).
Patch coverage: 80.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10340   +/-   ##
=======================================
  Coverage   93.85%   93.86%           
=======================================
  Files        1597     1597           
  Lines      145394   145394           
=======================================
+ Hits       136462   136467    +5     
+ Misses       8932     8927    -5     

Impacted Files	Coverage Δ
...egration_tests/golden/golden_kitchen_sink_client.h	100.00% <ø> (ø)
...ion_tests/golden/golden_kitchen_sink_connection.cc	30.30% <0.00%> (ø)
...tion_tests/golden/golden_kitchen_sink_connection.h	100.00% <ø> (ø)
...den/internal/golden_kitchen_sink_connection_impl.h	83.33% <ø> (ø)
...golden/mocks/mock_golden_kitchen_sink_connection.h	72.72% <ø> (ø)
generator/internal/connection_impl_generator.cc	48.48% <ø> (ø)
generator/internal/mock_connection_generator.cc	63.63% <ø> (ø)
generator/internal/client_generator.cc	26.94% <66.66%> (ø)
...gration_tests/golden/golden_kitchen_sink_client.cc	80.68% <100.00%> (ø)
...tor/integration_tests/golden/internal/streaming.cc	60.00% <100.00%> (ø)
... and 8 more

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