chore!: updates as Debian 9 (Stretch) is EOL

[coryan]

Debian 9 (Stretch) reached EOL on 2022-06-30. Therefore, we have stopped
testing or supporting this distribution. This was the last distribution
we supported that required GCC < 7.3, and/or CMake < 3.10. Starting with
this release we require GCC >= 7.3 and CMake >= 3.10.

This PR updates the documentation and CI builds.

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: dbcdd7b9aea6fed6476466605c86cc1925bbd859

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #9405 (01e5ab2) into main (d52655a) will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##             main    #9405   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files        1485     1484    -1     
  Lines      136071   136069    -2     
=======================================
+ Hits       128730   128731    +1     
+ Misses       7341     7338    -3     

Impacted Files	Coverage Δ
generator/internal/scaffold_generator.cc	83.59% <ø> (ø)
.../cloud/bigtable/internal/legacy_row_reader_test.cc	100.00% <ø> (ø)
...ge/tests/curl_download_request_integration_test.cc	92.47% <0.00%> (ø)
...oud/storage/tests/curl_request_integration_test.cc	96.76% <0.00%> (ø)
google/cloud/bigtable/internal/defaults_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/curl_handle_test.cc	100.00% <100.00%> (ø)
google/cloud/spanner/internal/defaults_test.cc	100.00% <100.00%> (ø)
google/cloud/storage/internal/curl_handle_test.cc	100.00% <100.00%> (ø)
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.11% <0.00%> (-0.08%)	⬇️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d52655a...01e5ab2. Read the comment docs.

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: bd4cabf2076fc7ed80ef0ad8b283eaf87a023e08

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: f531948b10b7488922f8d3095719b52959a81a48

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 01e5ab21e0163eb6faca11ecf4f285df8d93a78d

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.