Prepare for v2.0.0b1 release

- Update CHANGELOG.md
- Update to bundled capnproto-1.0.1
  * Compiles with capnproto-0.8.0 and higher
- *Breaking Change* Remove allow_cancellation (see
  https://capnproto.org/news/2023-07-28-capnproto-1.0.html)
  * This is tricky to handle for older versions of capnproto. Instead of
    dealing with lots of complication, removing it entirely.
- Fix some documentation after the build backend support was added
- Update tox.ini to support 3.8 to 3.12
- Update cibuildwheel to 2.16.1
  * Adds Python 3.12 supports and implicitly deprecates EOL 3.7 (though it's
    still built)

capnp/includes/capnp_cpp.pxd

@@ -476,7 +476,6 @@ cdef extern from "capnp/capability.h" namespace " ::capnp":
         # void adoptResults(Orphan<Results>&& value);
         # Orphanage getResultsOrphanage(uint firstSegmentWordSize = 0);
         VoidPromise tailCall(Request & tailRequest)
-        void allowCancellation() except +reraise_kj_exception
 
 cdef extern from "kj/async.h" namespace " ::kj":
     cdef cppclass EventPort: