[chore] Remove toolchain directive from cmd/otelcorecol (#14137)

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#### Description

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This is not really needed anymore in recent Go versions, so I think we
can remove it and avoid CI failures on every patch.

In particular, [the Go 1.25 release notes](https://go.dev/doc/go1.25)
state:

> When the go command updates the go line in a go.mod or go.work file,
it [no longer](https://go.dev/ref/mod#go-mod-file-toolchain) adds a
toolchain line specifying the command’s current version.

Author: mx-psi

.github/workflows/build-and-test.yml

@@ -93,7 +93,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@44694675825211faa026b3c33043df3e48a5fa00 # v6.0.0
         with:
-          go-version: oldstable
+          go-version: stable
           cache: false
       - name: Cache Go
         id: go-cache

cmd/otelcorecol/go.mod

@@ -4,8 +4,6 @@ module go.opentelemetry.io/collector/cmd/otelcorecol
 
 go 1.24.0
 
-toolchain go1.24.9
-
 require (
 	go.opentelemetry.io/collector/component v1.45.0
 	go.opentelemetry.io/collector/confmap v1.45.0