Spring Deprecate StringUtils::trimWhitespace

PR #27703 has made StringUtils::trimWhitespace trivial, as the method now delegates to String::strip. The only significance that trimWhitespace accepts null. We should deprecate trimWhitespace in favor of String::strip.

Similarly, we should deprecate StringUtils::trimLeadingWhitespace in favor of String::stripLeading, and StringUtils::trimTrailingWhitespace in favor of String::stripTrailing.