mpdecimal: Misc cleanup

[Ahajha]

Primary changes, roughly in order of LOC changed:

• Remove the 'configure' patches, these no longer have any effect since that script isn't run anymore.
• Remove the v1 test package
• Remove the legacy CONAN_***FLAGS being patched in
• Simplify the 'package' step (mainly unifying all the different cases)
• Simplify the 'build' step (mainly remove the exact target name calculation and letting the build system handle it)
• Simplify the 'generate' step (mainly remove redundant or unnecessary generators)
• Remove usage of the deprecated 'path' variables
• Add in patch descriptions
• Make patch file naming more consistent

There are some very minor functional differences:

• Fixed the license file not being packaged in 4.0.0, LICENSE.txt was changed to COPYRIGHT.txt
• The LIBMPDECXX_DLL define has been unified with LIBMPDEC_DLL - I see no reason to distinguish these two as they are either both shared or both static.

Related: 2.4.2 is being removed in #26092

[Ahajha]

@jcar87 This is still a draft, but the "Validate PR" job has been failing since the "Merge 2.4.2 into 2.5.x folder" commit. Does the new CI not like modifying multiple folders under the same recipe?