change mysql driver to mysql8 as mysql is no longer supported

[hungcq]

What was changed

change "mysql" driver in values.yaml and example values.aurora-mysql.yaml to "mysql8"

Why?

"mysql" is no longer in the list of supported drivers (cassandra, mysql8, postgres12, postgres12_pgx, elasticsearch)

Checklist

1. Closes
   This is a small change so I don't open an issue for it.

2. How was this tested:
   I deployed the helm chart, using sql as default/visibility DB.

3. Any docs updates needed?
   No, README doesn't contain this part.

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