Remove dependency to Spring Boot

[fmbenhassine]

As of v3.4, spring-shell-core depends on spring-boot-starter-validation and transitively on spring-boot-starter. Here is the dependency tree for a test project that depends only on the core jar:

>./mvnw dependency:tree
[INFO] Scanning for projects...
[INFO] org.springframework.shell:shell-test:jar:1.0-SNAPSHOT
[INFO] \- org.springframework.shell:spring-shell-core:jar:3.4.0:compile
[INFO]    +- org.springframework:spring-core:jar:6.2.0:compile
[INFO]    |  \- org.springframework:spring-jcl:jar:6.2.0:compile
[INFO]    +- org.springframework.boot:spring-boot-starter-validation:jar:3.4.0:compile
[INFO]    |  +- org.springframework.boot:spring-boot-starter:jar:3.4.0:compile
[INFO]    |  |  +- org.springframework.boot:spring-boot:jar:3.4.0:compile
[INFO]    |  |  +- org.springframework.boot:spring-boot-autoconfigure:jar:3.4.0:compile
[INFO]    |  |  +- org.springframework.boot:spring-boot-starter-logging:jar:3.4.0:compile
[INFO]    |  |  |  +- ch.qos.logback:logback-classic:jar:1.5.12:compile
[INFO]    |  |  |  |  +- ch.qos.logback:logback-core:jar:1.5.12:compile
[INFO]    |  |  |  |  \- org.slf4j:slf4j-api:jar:2.0.15:compile
[INFO]    |  |  |  +- org.apache.logging.log4j:log4j-to-slf4j:jar:2.24.1:compile
[INFO]    |  |  |  |  \- org.apache.logging.log4j:log4j-api:jar:2.24.1:compile
[INFO]    |  |  |  \- org.slf4j:jul-to-slf4j:jar:2.0.16:compile
[INFO]    |  |  +- jakarta.annotation:jakarta.annotation-api:jar:2.1.1:compile
[INFO]    |  |  \- org.yaml:snakeyaml:jar:2.3:compile
[INFO]    |  +- org.apache.tomcat.embed:tomcat-embed-el:jar:10.1.33:compile
[INFO]    |  \- org.hibernate.validator:hibernate-validator:jar:8.0.1.Final:compile
[INFO]    |     +- jakarta.validation:jakarta.validation-api:jar:3.0.2:compile
[INFO]    |     +- org.jboss.logging:jboss-logging:jar:3.4.3.Final:compile
[INFO]    |     \- com.fasterxml:classmate:jar:1.5.1:compile
[INFO]    +- org.springframework:spring-messaging:jar:6.2.0:compile
[INFO]    |  \- org.springframework:spring-beans:jar:6.2.0:compile
[INFO]    +- io.projectreactor:reactor-core:jar:3.7.0:compile
[INFO]    |  \- org.reactivestreams:reactive-streams:jar:1.0.4:compile
[INFO]    +- org.jline:jline-console:jar:3.26.3:compile
[INFO]    |  \- org.jline:jline-builtins:jar:3.26.3:compile
[INFO]    +- org.jline:jline-reader:jar:3.26.3:compile
[INFO]    +- org.jline:jline-style:jar:3.26.3:compile
[INFO]    +- org.jline:jline-terminal:jar:3.26.3:compile
[INFO]    |  \- org.jline:jline-native:jar:3.26.3:compile
[INFO]    +- org.antlr:ST4:jar:4.3.4:compile
[INFO]    |  \- org.antlr:antlr-runtime:jar:3.5.3:compile
[INFO]    +- commons-io:commons-io:jar:2.18.0:compile
[INFO]    \- org.springframework.shell:spring-shell-table:jar:3.4.0:runtime
[INFO]       \- org.springframework:spring-context:jar:6.2.0:compile
[INFO]          +- org.springframework:spring-aop:jar:6.2.0:compile
[INFO]          +- org.springframework:spring-expression:jar:6.2.0:compile
[INFO]          \- io.micrometer:micrometer-observation:jar:1.14.0:compile
[INFO]             \- io.micrometer:micrometer-commons:jar:1.14.0:compile
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  6.947 s
[INFO] Finished at: 2025-04-23T14:55:22+02:00
[INFO] ------------------------------------------------------------------------

The core jar should not depend on Boot. Only the autoconfigure module can depend on Boot.