fix: mysql version fix 8.0.3 -> 8.3.0

[dandawg]

Description

It Looks like the mysql:8.0.3 image version may have been a mistake, and it really should be mysql:8.3.0. I updated the mysql image version from 8.0.3 to 8.3.0 in the manifest file. This resolves three issues:

1. The README suggest version 8.3.0 is being used in some notes near the bottom (see https://github.com/dandawg/model-registry#pull-image-rate-limiting), which is inconsistent with what is in the manifest (8.0.3) as is.
2. The mysql:8.0.3 image results in ImagePullBackoff errors on Kind (at least on Mac). This can also be verified with docker pull mysql:8.0.3.
3. The mysql:8.0.x versions are older (7 years old!) and have more vulnerabilities, so it will be better to use 8.3.0.

Note: this should fix issue #266

How Has This Been Tested?

Limited testing on Kind on Mac M2. Once I updated the version, the deployment was able to succeed. The rest of the model registry pods/services/deployments also came up without issue.

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

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If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
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[google-oss-prow]

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[dandawg]

@rareddy @tarilabs I was just reading issue #266 and realized that this fixes that issue.