Add %kvp to default logging pattern to support SLF4J 2.x fluent key-value logging

[hanouard]

Description

Spring Boot 3 uses SLF4J 2.x, which introduced the fluent logging API (log.atInfo().addKeyValue(...).log(...)).
Currently, when using the default logging pattern, the key–value pairs are silently ignored because %kvp is not included in the log pattern.

This PR updates the default console and file logging pattern to append %kvp after the log message (%m).

Why

Developers using SLF4J’s fluent API expect key–value pairs to be visible without custom Logback configuration.

Aligns Spring Boot defaults with SLF4J 2.x features.

Provides more observability by making structured key–value logging visible out-of-the-box.

Changes

log.atInfo()
    .addKeyValue("userId", "42")
    .addKeyValue("action", "login")
    .log("User action performed");

Before:

2025-09-11 16:30:05.123  INFO --- [main] c.e.Demo : User action performed

After:

2025-09-11 16:30:05.123  INFO --- [main] c.e.Demo : User action performed userId="42" action="login"