Upgrade Cloud SQL Proxy to v2 for tests

[jackwotherspoon]

The testing infrastructure starts the Cloud SQL Proxy (v1) prior to running tests:

python-docs-samples/.kokoro/tests/run_tests.sh

Lines 134 to 145 in 8570053

	# Run Cloud SQL proxy (background process exit when script does)
	wget --quiet https://dl.google.com/cloudsql/cloud_sql_proxy.linux.amd64 \
	-O ${HOME}/cloud_sql_proxy && chmod +x ${HOME}/cloud_sql_proxy
	${HOME}/cloud_sql_proxy -instances="${MYSQL_INSTANCE}"=tcp:3306,"${MYSQL_INSTANCE}" -dir "${HOME}" &>> \
	${HOME}/cloud_sql_proxy.log &
	echo -e "\Cloud SQL proxy started for MySQL."
	${HOME}/cloud_sql_proxy -instances="${POSTGRES_INSTANCE}"=tcp:5432,"${POSTGRES_INSTANCE}" -dir "${HOME}" &>> \
	${HOME}/cloud_sql_proxy-postgres.log &
	echo -e "\Cloud SQL proxy started for Postgres."
	${HOME}/cloud_sql_proxy -instances="${SQLSERVER_INSTANCE}"=tcp:1433 &>> \
	${HOME}/cloud_sql_proxy-sqlserver.log &
	echo -e "\Cloud SQL proxy started for SQL Server."

The Cloud SQL Proxy (v2) was released on Jan 17, 2023. Latest is now v2.15.0 and is highly recommended over v1 usage.

The v2 has numerous new features and improvements over v1 such as debug logging,
lazy refresh, PSC connections, non-GDU environments, metrics and tracing, etc.

There is a migration guide that can be used to help with the CLI changes required to migrate.

Should also clean up any stale usage of v1 Proxy in this repo, which can be found by looking
for cloud_sql_proxy as the v2 Proxy binary is cloud-sql-proxy.