[jdbc] Reduce the default character limit for VARCHAR columns in MySQL

[jlaur]

See https://dev.mysql.com/doc/refman/8.3/en/storage-requirements.html and namely:

The effective maximum number of bytes that can be stored in a VARCHAR or VARBINARY column is subject to the maximum row size of 65,535 bytes, which is shared among all columns. For a VARCHAR column that stores multibyte characters, the effective maximum number of characters is less. For example, utf8mb4 characters can require up to four bytes per character, so a VARCHAR column that uses the utf8mb4 character set can be declared to be a maximum of 16,383 characters. See Section 10.4.7, “Limits on Table Column Count and Row Size”.

The old size was based on up to three bytes per character, but utf8mb4 can require up to four bytes per character. On top of that we need space for the time column as well. This is now aligned with MariaDB:

openhab-addons/bundles/org.openhab.persistence.jdbc/src/main/java/org/openhab/persistence/jdbc/internal/db/JdbcMariadbDAO.java

Lines 59 to 60 in 6ebf0f9

	sqlTypes.put("IMAGEITEM", "VARCHAR(16255)");
	sqlTypes.put("STRINGITEM", "VARCHAR(16255)"); // MariaDB using utf-8 max = 16383, using 16383-128 = 16255

Resolves #13920
Resolves #16678

[lsiepel]

LGTM