Add release note for nalgebra 2q synthesis changes

[mtreinish]

In the recently merged #16065 we added a new release note section to highlight performance changes that weren't additions. Since that has merged for the 2.5 release the PR series migrating our internal 2q synthesis code to use nalgebra should be documented under this section for the 2.5.0 release notes. Since those PRs (#15960 and #15928) merged before #16065 did there was no release note documenting this change. This commit corrects that oversight and adds the release note so we're documenting this improvement in the performance section.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code:

[qiskit-bot]

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

• @Qiskit/terra-core

[coveralls]

Coverage Report for CI Build 26493870714

Coverage decreased (-0.003%) to 87.602%

Details

• Coverage decreased (-0.003%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• 19 coverage regressions across 4 files.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

19 previously-covered lines in 4 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/parse.rs	12	97.15%
crates/circuit/src/parameter/symbol_expr.rs	4	74.09%
crates/qasm2/src/lex.rs	2	92.03%
crates/circuit/src/parameter/parameter_expression.rs	1	91.04%

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Coverage Stats

Relevant Lines:	123981
Covered Lines:	108610
Line Coverage:	87.6%
Coverage Strength:	960746.53 hits per line

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💛 - Coveralls

[ShellyGarion]

Thanks for adding the performance release notes!

1. Do you think it's worth also adding PR #15871?

2. In PR #16024, you've added features_misc saying that "This improves the runtime performance of Qiskit and also should reduce the resident set size when running Qiskit."
   whould you like to change it to "performance" ?

[mtreinish]

Do you think it's worth also adding PR #15871?

I didn't since it wasn't really about performance on it's own. It was in service of the note I added here. On it's own changing the internal return type doesn't do much because it would just use an ndarray for the synthesis. But when combined with the later PRs we get the performance improvement by doing all the matrix storage with nalgebra

In PR #16024, you've added features_misc saying that "This improves the runtime performance of Qiskit and also should reduce the resident set size when running Qiskit."
whould you like to change it to "performance" ?

I think it probably should be a performance release note. But it's kind of independent from this release note PR (which is adding the missing one for the nalgebra migration). I think that it's something we can do in the release note roundup when we prepare the final 2.5.0 release.

[ShellyGarion]

One minor fix and it's ready to go :)

I think it probably should be a performance release note. But it's kind of independent from this release note PR (which is adding the missing one for the nalgebra migration). I think that it's something we can do in the release note roundup when we prepare the final 2.5.0 release.

I'll try to remind the release manager of Qiskit 2.5 about this when preparing the release PR :)

[ShellyGarion]

I think it probably should be a performance release note. But it's kind of independent from this release note PR (which is adding the missing one for the nalgebra migration). I think that it's something we can do in the release note roundup when we prepare the final 2.5.0 release.

done as part of #16454