Document the JdbcAppender truncateStrings attribute

[dajiaohuang]

JdbcAppender.Builder#truncateStrings is a @PluginBuilderAttribute with a default of true, so
it can be configured on the <Jdbc> element. It was the only optional attribute of the appender
that the manual did not document, which makes the attribute undiscoverable even though it changes
what is written to the database: when it is true, values written to string columns are truncated
to the size reported by the database metadata.

This adds the missing row to the attribute table in
src/site/antora/modules/ROOT/pages/manual/appenders/database.adoc, in the same format as the
neighbouring reconnectIntervalMillis row.

Validation: rendered the page with Asciidoctor and confirmed the table still has four columns per
row and that the new row is emitted with the JdbcAppender-attr-truncateStrings anchor, the type
boolean and the default true. The full Antora build was not run locally; it is driven by the
shared apache/logging-parent deploy-site workflow.

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (the build instructions)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests are provided