community/mupdf: encode full pkgver in SONAME of shared library

Upstream provides no ABI stability across updates, hence we always need to rebuild mupdf even for patch version upgrades. Previously only the major + minor version was encoded in the SONAME (e.g. mupdf-1.23). This is insufficient as apk's dependency tracker will then not update dependency on partial upgrades.

This changeset also removes the libmupdf.so.1.23 symlink and only provides libmupdf.so → libmupdf.so.1.23.3.