* Handle single bit conditions in QPY

In #6018 initial support for adding classical conditions on a single bit
instead of a register was added. However this wasn't accounted for in
qpy because it didn't exist when qpy was first written. This commit adds
support to qpy without a file format change so it can be backported and
used with already released versions of qpy without needing a new format
version. This is accomplished by exploiting the strict register naming
in qiskit. A register can't have a name outside of regex
"[a-z][a-zA-Z0-9_]*" which we leverage in the case of single clbit
conditions the "register" name in the output QPY data is set to
str(clbit_index) which isn't a valid name. Then on the loading side we
check for a valid name, if it's outside the allowed regex we treat the
condition as a single bit and the name is a str(index). The tradeoff here
is that for a QPY file generated with 0.18.1 (assuming this is backported
and included in 0.18.1) this qpy file can not be loaded with qiskit
0.18.0. But this is fine as we only guarantee compatibility in one
direction (loading qpy files generated with older with qiskit with a
newer version of qiskit).

Fixes partially #6475

* Add release note

* Prefix bit index with null character

This commit modifies the special string we use in the register name
field to be prefixed with a null character. This leaves open using the
string for other special cases and also returning a sane error if an
invalid QPY file was generated by something besides Qiskit.

* Update qiskit/circuit/qpy_serialization.py

Co-authored-by: Jake Lishman <jake@binhbar.com>

* Add note to release note about feature not being supported

* Add backwards compat test case too

Co-authored-by: Jake Lishman <jake@binhbar.com>
Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit afb0c47)