Go SDK: a pointer spec field cannot be set from a literal #73951

What happens

airflow.DagSpec and airflow.TaskSpec carry 9 pointer fields, so that a value an author
sets explicitly is distinguishable from one left unset — a bool whose default comes from
Airflow config, such as catchup, is the clearest case.

Go does not allow taking the address of a literal, so the obvious spelling does not compile:

dag := airflow.Dag("etl", airflow.DagSpec{Catchup: &false})  // invalid operand

An author has to name a variable for every such field:

catchup := false
dag := airflow.Dag("etl", airflow.DagSpec{Catchup: &catchup})

What would fix it

Go 1.26's new(expr):

dag := airflow.Dag("etl", airflow.DagSpec{Catchup: new(false)})

Why this is deferred rather than done

go-sdk/go.mod is on go 1.25.0. Go 1.26 was released in February 2026, which is too new
to require of everyone who builds a Go bundle, so the floor is not being raised for this.
Decided while reviewing #73936.

An airflow.Ptr[T] generic helper was considered and rejected there: the pointer
transformation is the language's job, not the SDK's.

Acceptance

• Go 1.26 is an acceptable minimum for the Go SDK.
• go-sdk/go.mod raised to it.
• new(expr) used in the Dag authoring docs and examples, so the spelling an author
  copies is the short one.

Drafted-by: Claude Code (Opus 5) (no human review before posting)