Misc optimizations

[phax]

• Require Java 11 as the minimum - otherwise Jakarta and other stuff won't work properly
• Allow a validation of Zugferd documents without requiring a file - an InputStream is sufficient. This also reduces the amount of IO operations needed to perform the activities
• Make sure the Zugferd validation APIs work in the same way
• Make some general code improvements (constant instead of UTF-8; remove unnecessary code; remove unnecessary imports)
• Remove an additional dependency to the BigFileSearcher as the data was already present as a byte[] so the ByteArraySearcher is more efficient
• Removed unused variables and unused private members
• Removed unused casts
• Removed unnecessary duplicate "implements"

[jstaerk]

cosmetical question: debCN, debCurrentChild sounds like "deb"ug? setOwnTaxID etc will continue to work? You might have noticed that I would very much like to publish as (backward compatible) Mustang 2.12 not yet Mustang 3. I would like to close this branch and only accept the new ZUGFeRD version when release, if there are further bigger refactorings or changes maybe we could start a 2.13 branch.

[phax]

@jstaerk well, I just reused the deb variables, as a performance improvement - they all existed beforehand. Do you want me to rename them to not contain deb? The setOwnTaxID methods etc. are of course still working. They are just marked as deprecated, so I replaced them with functionally-equal non-deprecated methods :) Hope that solves your questions.