simple-phonebook-web-application with MySQL versión 8

Hello.

Starting with MySQL8, the default character code seems to be utf8mb4, so I think this is probably the problem.
Therefore, try changing the character code to utf8 in the parameter group set in the RDS instance, or change the character code of the table created within RDS to utf8.
https://stackoverflow.com/questions/43437490/pdo-construct-server-sent-charset-255-unknown-to-the-client-please-rep

I think the answers below will be helpful.
https://stackoverflow.com/a/58959536

For setting MySQL's character code, please check the parameters introduced in the blog below.
It is explained in detail in "Parameters related to character sets".
https://aws.amazon.com/jp/blogs/database/best-practices-for-configuring-parameters-for-amazon-rds-for-mysql-part-3-parameters-related-to-security-operational-manageability-and-connectivity-timeout/