Use java.util.Base64 instead of BASE64Encoder

[danielcompton]

For backwards/forwards compatibility between implementations, adds a
trailing newline to the string to match the behaviour of the sun.misc.BASE64Decoder.

This effectively sets the minimum JVM to Java 8, at least to use this
namespace.

[puredanger]

Based on data from the 2018 State of Clojure (almost a year ago), 93% of the community was using Java 8 or higher. I expect that number is higher now.

[danielcompton]

Thanks @puredanger that's very helpful information. I had thought about asking a question on the mailing list but had forgotten about the survey data. I'm leaning towards merging this then, but will give it a little bit more time for others to comment also.

[NoahTheDuke]

If this is going to be merged, can the existing reliance on the sun.misc.* stuff be removed? While it's still in, the library is unusable from java 11+.