Extend license header copyright years to 2023.

See spring-projects#2476

Author: mp911de

src/main/asciidoc/index.adoc

@@ -8,7 +8,7 @@ ifdef::backend-epub3[:front-cover-image: image:epub-cover.png[Front Cover,1050,1
 :spring-framework-reference: https://docs.spring.io/spring-framework/docs/{springVersion}/reference/html
 :store: Redis
 
-(C) 2011-2022 The original authors.
+(C) 2011-2023 The original authors.
 
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