remove old deprecations and other basic housekeeping

[ExpandingMan]

This includes a whole bunch of basic housekeeping maintenance, centering mostly around removing deprecations that have been in the package for a very long time.

Summary of Changes:

• All deprecations have been removed, including their unit tests. (They were all very old.)
• Support for versions older than 1.6 has been removed. These were mostly @static check blocks.
• ConstructionBase has been added as a dependency. Previously there was a dubious hack that created this as a "fake" module to accommodate older versions. Since this package defines methods for ConstructionBase and it is already an indirect dependency, we should add it as a direct dependency.
• The Manifest.toml for Documenter has been updated and now uses the local version of Transducers.
• This section of the docs has been removed entirely. It is entirely dedicated to discussing a change which is now 2 years old according to commit logs.
• In Literate.jl examples, some code blocks which were previously interspersed with normal documentation have been removed, with the documentation text now as inline comments. I personally find the split up blocks almost unreadable.
• I've updated the benchmark Manifest. I'm a bit confused as to how that doesn't seem to have been referring to whatever is the local version of Transducers.
• CI/CD stuff has been updated to default to 1.9 where applicable.
• Fixed various CI/CD URL's.
• Fixed various deprecations in benchmarks, mostly related to |>.
• Added a function to docs to remove files generated by Literate.jl. This is useful if you are testing changes that include moving or removing files, otherwise the files may persist regardless of whether they've been moved in examples.
• Comment out addprocs(4) in exmples/tutorials/tutorial_parallel.jl. Previously this was marked with #md which indicates to Literate.jl not to read it, however Documenter.jl was still trying to evaluate it and was hanging. This might be a bug in either Documenter.jl or Literate.jl because I'd expect that even if Documenter doesn't understand #md it should think it's a comment.
• Fix links in the README that were referring to JuliaFolds instead of JuliaFolds2, as in updated README links to refer to JuliaFolds2 #8 (can be closed).
• Many small fixes for doc tests, mostly to address subtle breaking changes in Documenter and in Julia itself.

Things I considered but did not change:

• I had originally intended to re-factor some of the opcompose in the docs, but there turned out to be a lot less of this than I thought. There is probably some stuff that's arguably still rather non-idiomatic, but nothing that I deemed worth including with all these other changes.
• I was pleasantly surprised to see that the list of transducers in the home page of the docs is already automated. The entries that I believed were omitted were not actually subtypes of Transducer. It would be nice to eventually include more tables similar to this, but I did not want to compromise the purity of the existing table, so I'm leaving this be for now.

[MasonProtter]

Thank you for this @ExpandingMan !