Avoid raising errors on database passwords that contain a $ character

[desertaxle]

This PR fixes errors raised when a database password contains a $ character by using safe_substitute when templating. Templating will not occur when a $ character is present in the password, which means that errors may still occur with settings like the following:

PREFECT_API_DATABASE_NAME="the-database"
PREFECT_API_DATABASE_CONNECTION_URL="postgresql+asyncpg://the-user:the-$password@localhost:5432/${PREFECT_API_DATABASE_NAME}"

Since this setup would fail without these changes, and the docs recommend setting PREFECT_API_DATABASE_CONNECTION_URL directly, I think this behavior is acceptable.

Closes #11067

Checklist

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[codspeed-hq]

CodSpeed Performance Report

Merging #14876 will not alter performance

_{Comparing fix-dbconnection-string-templating (3f7d100) with main (1259703)}

Summary

✅ 4 untouched benchmarks

[cicdw]

Makes sense - I think we can eventually find a way to support both $ characters and variable expansion in the same setting, but given that this implementation is a strict improvement we can consider that in the future.

[zzstoatzz]

lgtm!