Typo in Kafka configuration (`keyStore` vs. `keystore`)

[snicoll]

When #13031 got merged, I failed to notice a typo in the property name. While changing that in a maintenance release can be seen as a breaking change, we'll still bind both though the current form will be seen as unknown on upgrade.

[snicoll]

Actually, the format in the PR is correct but the other keys introduced in #6961 are in the wrong format compared to other SSL related properties.

I think we need to harmonize them but I am not sure if that needs to happen in 2.0.x now.

[snicoll]

We've decided to keep that inconsistency now that 2.0.2 is released and rename all those properties to use keyStore for consistency with other SSL-related properties in Spring Boot.

[snicoll]

I forgot to add error level metadata for the previous properties