…olimits in `AddCommGrp` (#20474)

The old version of this file has a remark saying "TODO: In fact, in `AddCommGrp` there is a much nicer model of colimits as quotients of finitely supported functions, and we really should implement this as well (or instead)."

This PR does precisely this (choosing the "instead" option). In fact, given a functor `F : J ⥤ AddCommGrp.{w}`, it constructs a candidate for the colimit of `F` as a quotient of `DFinsupp (fun j ↦ F.obj j)`, without any smallness assumptions on `J`. It then proves that this quotient is a colimit of `F` if it is `w`-small, then deduces that `AddCommGrp.{w}` has all small colimits.




Co-authored-by: smorel394 <67864981+smorel394@users.noreply.github.com>

* origin/master: (189 commits)
  chore(Algebra): make more names consistent (#20449)
  feat: Polynomial FLT (#18882)
  feat: A disjoint union is finite iff all sets are finite, and all but finitely many are empty (#20457)
  fix: linkfix in 100.yaml (#20517)
  feat(1000): fill in more entries (#20470)
  feat(Combinatorics/SimpleGraph/Walk): add `penultimate` and `snd` (#16769)
  feat(ContinuousMultilinearMap): add lemmas about `.prod` (#20462)
  feat(RingTheory): classification of etale algebras over fields (#20324)
  fix: Allow multiple builds on staging branch (#20515)
  feat(CategoryTheory/Shift/Adjunction): properties of `Adjunction.CommShift` (#20337)
  chore(Data/Multiset/Basic): move the `sub`, `union`, `inter` sections lower (#19863)
  chore(Topology/UniformSpace/Completion): make coe' argument implicit (#20514)
  refactor(Algebra/Category/Grp/Colimits): change the construction of colimits in `AddCommGrp` (#20474)
  feat(Topology/Algebra/InfiniteSum/NatInt): Add pnat lems (#16544)
  chore(RingTheory/Finiteness): rename Module.Finite.out (#20506)
  refactor(CategoryTheory/Limits/Preserves/Ulift): simplify the proof that the universe lifting functor for types preserves all colimits (#20508)
  feat(CategoryTheory): products in the category of Ind-objects (#20507)
  chore: remove >9 month old deprecations (#20505)
  chore(FieldTheory/Galois): small cleanup (#20217)
  chore(LinearIndependent): generalize to semirings (#20480)
  chore(NumberTheory/NumberField/AdeleRing): refactor adele rings (#20500)
  chore: replace `aesop` with `simp` where possible (#20483)
  chore(1000): remove nonexistent fields (#20493)
  chore(CategoryTheory/Adjunction.Basic, CategoryTheory/Equivalence): change definition of `Adjunction.comp` and `Equivalence.trans` (#20490)
  feat(Asymptotics): add `Asymptotics.*.*Prod` lemmas (#20458)
  feat: a conditional kernel is almost everywhere a probability measure (#20430)
  feat: if `f` is a measurable group hom, then every point has a neighborhood `s` such that `f '' s` is bounded (#20304)
  feat: `Ico`, `Ioc`, and `Ioo` are not closed or compact (#20479)
  chore: drop redundant hypothesis in `Module.Dual.eq_of_preReflection_mapsTo` (#20492)
  feat(FaaDiBruno): derive `DecidableEq` (#20482)
  chore(SetTheory/Ordinal/Basic): `{x // x < y}` → `Iio y` (#20413)
  chore: generalize `FormalMultilinearSeries.ofScalars_norm` to `Seminormed` (#20351)
  chore(MvPolynomial/Localization): golf using TensorProduct (#20309)
  chore(Combinatorics/Enumerative/Partition): auto-derive DecidableEq (#20277)
  chore(CategoryTheory): make relevant parameters explicit in `Arrow.homMk`. (#20259)
  feat: add `IsStarNormal (↑x⁻¹ : R)` instance for `x : Rˣ` (#20091)
  fix: Stop cancelling builds of master (#20435)
  chore: update Mathlib dependencies 2025-01-05 (#20485)
  feat(SetTheory/Cardinal/Arithmetic): miscellaneous cardinality lemmas (#18933)
  chore: bump toolchain to v4.16.0-rc1, and merge bump/v4.16.0 (#20464)
  chore: bot validates lean-toolchain on bump/v4.X.0 branches (#20478)
  feat: shorthand lemmas for the L1 norm (#20383)
  chore: remove unnecessary adaptation notes (#20467)
  chore(Algebra/Category/MonCat/Colimits): remove smallness condition (#20473)
  chore(SetTheory/Ordinal/Arithmetic): `IsLeftCancelAdd Ordinal` (#19888)
  feat(Algebra): more on `OrthogonalIdempotents` (#18195)
  feat(Radical): `(radical a).natDegree ≤ a.natDegree` (#20468)
  feat(Polynomial): `(a^n)' = 0` iff `a' = 0` when `n` doesn't divide the characteristic (#20190)
  feat(Algebra/Lie): add Lie's theorem (#13480)
  chore(RingTheory/Generators): make algebra instance a def (#14265)
  ...