Add spring.web.resources.cache.use-last-modified

Prior to this commit, packaging a Spring Boot application as a container
image with Cloud Native Buildpacks could result in unwanted browser
caching behavior, with "Last-Modified" HTTP response headers pointing to
dates in the far past.

This is due to CNB resetting the last-modified date metadata for static
files (for build reproducibility and container layer caching) and Spring
static resource handling relying on that information when serving static
resources.

This commit introduces a new configuration property
`spring.web.resources.cache.use-last-modified` that can be used to
disable this behavior in Spring if the application is meant to run as a
container image built by CNB.

The default value for this property remains `true` since this remains
the default value in Spring Framework and using that information in
other deployment models is a perfectly valid use case.

Fixes gh-24099

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -8925,7 +8925,9 @@ This means you can just type a single command and quickly get a sensible image i
 
 Refer to the individual plugin documentation on how to use buildpacks with {spring-boot-maven-plugin-docs}#build-image[Maven] and {spring-boot-gradle-plugin-docs}#build-image[Gradle].
 
-
+NOTE: In order to achieve reproducible builds and container image caching, Buildpacks can manipulate the application resources metadata (such as the file "last modified" information).
+You should ensure that your application does not rely on that metadata at runtime.
+Spring Boot can use that information when serving static resources, but this can be disabled with configprop:spring.web.resources.cache.use-last-modified[]
 
 [[boot-features-whats-next]]
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