fix: interface proto libraries work with older CMake

[coryan]

Older versions of CMake are (too) strict about what properties may be attached to an interface library. That was not working for some functions we use with proto libraries.

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This change is 

[codecov]

Codecov Report

Base: 93.87% // Head: 93.86% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (48f15b9) compared to base (ef0e4f8).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10636      +/-   ##
==========================================
- Coverage   93.87%   93.86%   -0.01%     
==========================================
  Files        1667     1667              
  Lines      150048   150048              
==========================================
- Hits       140853   140849       -4     
- Misses       9195     9199       +4     

Impacted Files	Coverage Δ
google/cloud/internal/async_connection_ready.cc	89.36% <0.00%> (-4.26%)	⬇️
...bigtable/examples/bigtable_hello_instance_admin.cc	81.00% <0.00%> (-2.00%)	⬇️
google/cloud/internal/timer_queue.cc	96.10% <0.00%> (-1.30%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	98.78% <0.00%> (-1.22%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	88.62% <0.00%> (-0.67%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	84.21% <0.00%> (-0.66%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.16% <0.00%> (-0.34%)	⬇️
google/cloud/pubsub/samples/samples.cc	87.92% <0.00%> (-0.08%)	⬇️
google/cloud/bigtable/async_read_stream_test.cc	97.99% <0.00%> (+0.66%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️
... and 1 more

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