[xlnt] update to 1.6.0

[m7913d]

Library name

xlnt

New version number

1.6.0

Other information that may be useful (release notes, etc...)

As discussed in issue tfussell/xlnt#748, the main repo of xlnt has been unmaintained for several years.
In order to continue this great project started by tfussell, the xlnt community has created a new repo to support further development by the community.

We are planning to release a new version (1.6.0) soon, mostly containing bug fixes and support for the latest toolchains. The changes are source-code backwards compatible with the 1.5.0 xlnt release of tfussell.
I'm already posting this issue now, to be able to take your feedback into account, before actually releasing the new xlnt version:

• Would you accept a repo switch from tfussell to xlnt-community, as the original repo is unmaintained? Advantage of reusing the xlnt port in vcpkg, is that all vcpkg users automatically gets the updated version.
• Or should we create a new port?