gnu: packages/golang: Set default go version to 1.21.

Golang versions 1.17 (02 Aug 2022), 1.18 (01 Feb 2023), 1.19 (06 Sep
2023) and 1.20 (06 Feb 2024) are EOL <https://endoflife.date/go>.

These changes set preferable default version to the minimal currently
maintained one e.g. 1.21

* gnu/packages/golang.scm (go): Set default to go-1.21, add annotation
for the future maintenance.

Remove field <#:go> setting Golang version to go-1.18, go-1.19, 1.20 and
1.21:
* gnu/packages/admin.scm
* gnu/packages/configuration-management.scm
* gnu/packages/databases.scm
* gnu/packages/disk.scm
* gnu/packages/education.scm
* gnu/packages/file-systems.scm
* gnu/packages/finance.scm
* gnu/packages/golang-check.scm
* gnu/packages/golang-crypto.scm
* gnu/packages/golang-web.scm
* gnu/packages/golang-xyz.scm
* gnu/packages/golang.scm
* gnu/packages/high-availability.scm
* gnu/packages/ipfs.scm
* gnu/packages/irc.scm
* gnu/packages/messaging.scm
* gnu/packages/networking.scm
* gnu/packages/syncthing.scm
* gnu/packages/textutils.scm
* gnu/packages/time.scm
* gnu/packages/version-control.scm
* gnu/packages/video.scm

Change-Id: Ieba6a5d7a18423f838d14c02ad55a2ba9c8559eb

gnu/packages/version-control.scm

@@ -3825,7 +3825,6 @@ If several repos are related, it helps to see their status together.")
     (arguments
      (list
       #:install-source? #f
-      #:go go-1.21
       #:import-path "github.com/x-motemen/ghq"
       #:phases
       #~(modify-phases %standard-phases