feat(Spanner): add sample for update database

[vishwarajanand]

Adding samples for update database RPC, providing enable drop protection for databases.

Note: tests will fail unless feature PR is merged & released: googleapis/google-cloud-php#6008

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• spanner/src/update_database.php:26, tag spanner_update_database

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[bshaffer]

Still getting the following error:

There was 1 error:

1) Google\Cloud\Samples\Spanner\spannerTest::testUpdateDatabase
Error: Cannot use object of type Google\ApiCore\OperationResponse as array

/tmpfs/src/github/php-docs-samples/spanner/src/update_database.php:52
/tmpfs/src/github/php-docs-samples/testing/sample_helpers.php:59
/tmpfs/src/github/php-docs-samples/spanner/src/update_database.php:59
/tmpfs/src/github/php-docs-samples/testing/vendor/google/cloud-tools/src/TestUtils/TestTrait.php:134
/tmpfs/src/github/php-docs-samples/testing/vendor/google/cloud-tools/src/TestUtils/TestTrait.php:146
/tmpfs/src/github/php-docs-samples/testing/vendor/google/cloud-tools/src/TestUtils/TestTrait.php:104
/tmpfs/src/github/php-docs-samples/spanner/test/spannerTest.php:1123
/tmpfs/src/github/php-docs-samples/spanner/test/spannerTest.php:247
/tmpfs/src/github/php-docs-samples/testing/vendor/bshaffer/phpunit-retry-annotations/src/RetryTrait.php:33

[vishwarajanand]

@bshaffer fixed the tests