Switch go default version to go 118

Go1.18 is the next stable go release. There is already Go1.19 (released 2022-08-02), but we incrementally increase versions. For Go1.18 there are already many bug fix releases (last one 2022-12-06) which gives it a more stable/mature impression.

Cc'ing people who might be affected:

• caddy (@luke)
• hugo (@lu.xu)
• repman, restic (@alain.takoudjou, @jerome)
• galene (@tomo, @alain.takoudjou)
• gitlab (@alain.takoudjou, @jerome, @lpgeneau)
• grafana (@jerome)
• theia (@jerome, @xavier_thompson)

(quoted from https://lab.nexedi.com/nexedi/slapos/commit/dae1747df9dab68288ccbdae9490595b44733f15)

Tests are pending: https://erp5js.nexedi.net/#/test_result_module/20221220-103AD325A

(I'll ping again once they are finished.)

/cc @kirr