Fix result.to_dict() following new 2.0 typing (backport #14124)

[mergify]

Following on the changes in #13793, I think we should also stop trying to serialize into a dictionary since the header should already be a dictionary.

Currently we face the following issue when doing result.to_dict()

  File "/opt/app-root/lib64/python3.11/site-packages/qiskit/result/result.py", line 91, in to_dict
    "header": None if self.header is None else self.header.to_dict(),
                                               ^^^^^^^^^^^^^^^^^^^
AttributeError: 'dict' object has no attribute 'to_dict'

Added corresponding regression test based on existing to_repr test

---

This is an automatic backport of pull request #14124 done by Mergify.

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 14171371221

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 88.136%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	3	92.98%

Totals
Change from base Build 14139800024:	0.02%
Covered Lines:	72811
Relevant Lines:	82612

---

💛 - Coveralls