Update javadoc to conform to JDK8's styling [SPR-11412]

[spring-projects-issues]

Rob Winch opened SPR-11412 and commented

JDK8's javadoc was updated and no longer has the resources folder. This caused issues with the customizations made for #14788 Specifically the customizations in commit 1c72406 relied on background images existing to ensure that the white banner was visible. JDK removed this image and thus the javadoc's banner was white rather than displaying the background image.

We should update the styling to conform to JDK8's defaults with only the customizations we need.

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Affects: 4.0.1

Referenced from: commits 14616a4