Rename version to version.txt

Rename version to version.txt

Building Notmuch on macOS is known to cause problems because the Notmuch
distribution archive contains two files named "version". These names
clash with the <version> header as defined in C++20. Therefore, the
existing naming will likely become a problem on other platforms as well,
once compilers adopt the new standard.

Signed-off-by: Ralph Seichter <github@seichter.de>
Amended-by: db s/keyword/header/ in commit message.