[bitnami/mysql] Add a warning on running older versions of MySQL

[fabiocarneiro]

Description of the change

Add a warning on the README section that explains that the chart is only tested with MySQL 8.x and will most likely not work with older versions.

Benefits

People will not waste time trying to run with older images or at least be aware that there is some additional research to be made on the differences and potential other changes required.

Possible drawbacks

None

Applicable issues

• fixes Make the chart mysql working with mysql 5.7 version #14827

Checklist

• Title of the pull request follows this pattern [bitnami/<name_of_the_chart>] Descriptive title
• All commits signed off and in agreement of Developer Certificate of Origin (DCO)

[carrodher]

Thanks for this contribution.

[carrodher]

Suggested change

	### Use a different MySQL 8.x version
	To modify the application version used in this chart, specify a different version of the image using the `image.tag` parameter and/or a different repository using the `image.repository` parameter.
	> **Warning**: This chart is tested with MySQL version 8.x. Older versions most likely won't work if you merely switch the tag.
	### Use a different MySQL version
	To modify the application version used in this chart, specify a different version of the image using the `image.tag` parameter and/or a different repository using the `image.repository` parameter.
	> **Warning**: Each bitnami/mysql Helm chart is created using a specific MySQL version which is tested in our automated test & release pipeline. Substituting the version could cause degraded performance, broken chart features, or missing environment variables.

I would opt for a more generic wording not only about 8.x

[github-actions]

This Pull Request has been automatically marked as "stale" because it has not had recent activity (for 15 days). It will be closed if no further activity occurs. Thank you for your contribution.