Modernize for Go 1.26

[brandur]

Follows up #1356 to bring back in some modernizers that I'd disabled to
not make the diff for that original PR gigantic.

In particular:

• errorsastype: Requires the use of the new errors.AsType helper.
  This is a really nice one and turns this old code:

    var pgErr *pgconn.PgError
    if errors.As(err, &pgErr) {
  

  Into this much nicer generics variant:

    if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok {
  

• newexpr: Converts pointer helpers like ptrutil.Ptr(...) to new.
  I remove all uses of ptrutil.Ptr(...), but left its definition in
  rivershared for now because I think it'll allow a little more
  flexibility in what version of River a River Pro installation can use.
  For example, an older version of River Pro could potentially point to
  a newer version of River and still have compilation work (it has
  however been marked as deprecated). I doubt too many people are in
  this situation though so we can probably remove it soon.

• stditerators: Replaces some manual iteration patterns with newer
  standard library iterator APIs. For example this:

    for i := range typ.NumField() {
        field := typ.Field(i)
  

  Goes to this:

    for field := range typ.Fields() {�[27;5;106~
  

[brandur]

thx!