main/tmux: upgrade to 3.1a

minor release with this in CHANGES file:
Do not close stdout prematurely in control mode since it is needed to
print exit messages. Prevents hanging when detaching with iTerm2.
7 jobs for !7351 with tmux-3.1a in 41 seconds
detached
Status Job ID Name Coverage
  Lint
failed #107448
docker-alpine x86_64 allowed to fail
lint

00:00:08

 
  Build
failed #107453
docker-alpine ci-build aarch64
build-aarch64

00:00:07

failed #107454
docker-alpine ci-build armv7
build-armv7

00:00:22

failed #107452
docker-alpine ci-build ppc64le
build-ppc64le

00:00:08

failed #107451
docker-alpine ci-build s390x
build-s390x

00:00:32

failed #107450
docker-alpine ci-build x86
build-x86

00:00:07

failed #107449
docker-alpine ci-build x86_64
build-x86_64

00:00:06

 
Name Stage Failure
failed
lint Lint
No job log
failed
build-x86 Build
No job log
failed
build-x86_64 Build
No job log
failed
build-aarch64 Build
No job log
failed
build-ppc64le Build
No job log
failed
build-armv7 Build
No job log
failed
build-s390x Build
No job log