New version: Manopt v0.4.61

[JuliaRegistrator]

• Registering package: Manopt
• Repository: https://github.com/JuliaManifolds/Manopt.jl
• Created by: @kellertuer
• Version: v0.4.61
• Commit: 77c672042d41657af9d8d95e37bc972acd96ed70
• Reviewed by: @kellertuer
• Reference: JuliaManifolds/Manopt.jl@77c6720#commitcomment-141429682
• Description: 🏔️Manopt. jl – Optimization on Manifolds in Julia
• Release notes:

### Added

* Tests now also use `Aqua.jl` to spot problems in the code, e.g. ambiguities.
* introduce a feature-based list of solvers and reduce the details in the alphabetical list
* adds a `PolyakStepsize`
* added a `get_subgradient` for `AbstractManifoldGradientObjectives` since their gradient is a special case of a subgradient.

### Fixed

* `get_last_stepsize` was defined in quite different ways that caused ambiguities. That is now internally a bit restructured and should work nicer.
  Internally this means that the interims dispatch on `get_last_stepsize(problem, state, step, vars...)` was removed. Now the only two left are `get_last_stepsize(p, s, vars...)` and the one directly checking `get_last_stepsize(::Stepsize)` for stored values.
* we accidentally exported `set_manopt_parameter!`, this is now fixed.

### Changed

* `get_manopt_parameter` and `set_manopt_parameter!` have been revised and better documented,
  they now use more semantic symbols (with capital letters) instead of direct field access
  (lower letter symbols). Since these are not exported, this is considered an internal, hence non-breaking change.
  * semantic symbols are now all nouns in upper case letters
  * `:active` is changed to `:Activity`

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