backports for 1.7.2

[benlorenz]

• read_metadata: use JSON for reading metadata to preserve order #5914
• Fix fitting_ideal to always return an ideal, even if it is the ideal generate by 1 (where sometimes the ring was returned instead) #5903
• Support null type in serialization schema, for future work on the OscarDB #5904
• Fix a bug that could prevent loading some serialized data #5906
• Fix documentation for polyhedral_fan to give the correct argument order #5917
• Fix parent of the generators of the ideal returned from groebner_basis_f4 when used with eliminate > 0 #5918

x-ref: #5919