cleanup: deprecate some IAM types

[coryan]

These types are only used in (already) deprecated IAM functions. I
wanted to deprecate each type, but that requires too many changes to
disable the diagnostics where we use the types. Instead I updated the
documentation and used deprecation attributes in the functions a
customer would use if they used the type.

Part of the work for #5929

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This change is 

[codecov]

Codecov Report

Merging #6069 (06e1c16) into master (a935d32) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6069      +/-   ##
==========================================
- Coverage   95.79%   95.79%   -0.01%     
==========================================
  Files        1167     1167              
  Lines      104149   104155       +6     
==========================================
+ Hits        99772    99777       +5     
- Misses       4377     4378       +1     

Impacted Files	Coverage Δ
google/cloud/iam_bindings.cc	95.55% <ø> (ø)
google/cloud/iam_bindings_test.cc	98.07% <ø> (ø)
google/cloud/iam_policy.h	100.00% <ø> (ø)
google/cloud/storage/internal/bucket_requests.cc	100.00% <ø> (ø)
...gle/cloud/storage/internal/bucket_requests_test.cc	97.65% <ø> (ø)
google/cloud/version.h	100.00% <ø> (ø)
google/cloud/iam_binding.h	100.00% <100.00%> (ø)
google/cloud/iam_bindings.h	96.00% <100.00%> (+1.26%)	⬆️
google/cloud/spanner/samples/samples.cc	84.30% <0.00%> (-0.15%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.83% <0.00%> (ø)
... and 1 more

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