Closes gh-26122

Closes gh-26121

Update `MavenPublishingConventions` to add a new `mavenOptional` feature
that allows us to declare optional dependencies that are also published
in the generated POM.

This change allows us to include the maven-shade-plugin in the
spring-boot-maven-plugin POM which fixes an issue with Eclipse m2e.

Fixes gh-21992

Fixes gh-23098

Closes gh-26113

Closes gh-26112

Closes gh-24741

Closes gh-25323

Closes gh-26111

Closes gh-26110

Closes gh-24027

Closes gh-25928

Closes gh-26109

Closes gh-26108

Closes gh-26084

Closes gh-26028

Closes gh-26053

Closes gh-25910

Closes gh-25905

Closes gh-26107

Closes gh-26105

* pr/26085:
  Polish "Document spring.config.additional-location should be used early"
  Document spring.config.additional-location should be used early

Closes gh-26085

See gh-26085

See gh-26085

Closes gh-26104

Closes gh-26103

Closes gh-26102

Closes gh-26101

Closes gh-26100

Closes gh-26099

Closes gh-26098

Closes gh-26097

Closes gh-26096