all: fix deprecated code and small lint issues detected by staticcheck

[peczenyj]

by running staticcheck ./... I found several small issues including

• usage of deprecated ioutil package
• grpc.Insecure() option
• explicit temp dir creation inside tests -- instead use t.TempDir()
• deprecated amqp Publish instead PublishWithContext
• and few other small changes

should not change any existing functionality.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 42.42424% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 73.30%. Comparing base (9cc772b) to head (5de446d).

Files	Patch %	Lines
pubsub/gcppubsub/gcppubsub.go	0.00%	3 Missing :warning:
samples/guestbook/gcp/deploy/main.go	0.00%	3 Missing :warning:
docstore/awsdynamodb/dynamo.go	50.00%	2 Missing :warning:
docstore/gcpfirestore/fs.go	0.00%	2 Missing :warning:
samples/guestbook/gcp/provision_db/main.go	0.00%	2 Missing :warning:
samples/guestbook/localdb/main.go	0.00%	2 Missing :warning:
aws/rds/rds.go	0.00%	1 Missing :warning:
azure/azuredb/azuredb.go	0.00%	1 Missing :warning:
internal/releasehelper/releasehelper.go	0.00%	1 Missing :warning:
samples/guestbook/aws/provision_db/main.go	0.00%	1 Missing :warning:
... and 1 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3438 +/- ## ========================================== - Coverage 73.32% 73.30% -0.03% ========================================== Files 113 113 Lines 14937 14939 +2 ========================================== - Hits 10953 10951 -2 - Misses 3206 3209 +3 - Partials 778 779 +1 ```

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[vangent]

Thanks for the cleanup!