improve mysql experimental warning
Changes the experimental warning from
ExperimentalWarning: "__init__" is an experimental function. It may break in future versions, even between dot releases.
ExperimentalWarning: "MySQLEventLogStorage" is an experimental class. It may break in future versions, even between dot releases.
Test Plan: ran bk, saw warnings section
Reviewers: sidkmenon, rexledesma
Reviewed By: rexledesma
Differential Revision: https://dagster.phacility.com/D6812