all: update standard-library dependencies at the start and end of each development cycle

[bcmills]

The x dependencies vendored into the standard library (via src/go.mod and src/cmd/go.mod) should be updated for each code freeze, so that we can apply any needed fixes to those dependencies without also pulling in unnecessary changes or requiring significant backporting work (see, for example, #36851).

This task, like #11811, #12042, and the API audit (#36167, #32813), should occur regularly in each development cycle. Probably we should update the dependencies at the beginning of each code freeze, and again when we reopen the tree at the end of each code freeze.

CC @golang/osp-team

[gopherbot]

Change https://golang.org/cl/217517 mentions this issue: all: update module dependencies

[dmitshur]

This issue is currently labeled as early-in-cycle for Go 1.15. That time is now, so friendly ping.

[bcmills]

Completed the early-in-cycle update for the 1.15 cycle. Removing the label so we remember to revisit at the close of the cycle.

[gopherbot]

Change https://golang.org/cl/231657 mentions this issue: all: update module dependencies

[dmitshur]

Completed the start-of-freeze update for the 1.15 cycle in CL 231657. Moving to 1.16 milestone and adding early-in-cycle.