Remove go 1.15 support (#18511)

- Remove support for go 1.15(go.mod already requires go 1.16).

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

modules/git/command.go

@@ -165,10 +165,6 @@ func (c *Command) RunWithContext(rc *RunContext) error {
 		"GIT_TERMINAL_PROMPT=0",
 	)
 
-	// TODO: verify if this is still needed in golang 1.15
-	if goVersionLessThan115 {
-		cmd.Env = append(cmd.Env, "GODEBUG=asyncpreemptoff=1")
-	}
 	cmd.Dir = rc.Dir
 	cmd.Stdout = rc.Stdout
 	cmd.Stderr = rc.Stderr

modules/git/git.go

@@ -35,9 +35,6 @@ var (
 
 	gitVersion *version.Version
 
-	// will be checked on Init
-	goVersionLessThan115 = true
-
 	// SupportProcReceive version >= 2.29.0
 	SupportProcReceive bool
 )
@@ -156,14 +153,6 @@ func Init(ctx context.Context) error {
 		return err
 	}
 
-	// Save if the go version used to compile gitea is greater or equal 1.15
-	runtimeVersion, err := version.NewVersion(strings.TrimPrefix(runtime.Version(), "go"))
-	if err != nil {
-		return err
-	}
-	version115, _ := version.NewVersion("1.15")
-	goVersionLessThan115 = runtimeVersion.LessThan(version115)
-
 	// Git requires setting user.name and user.email in order to commit changes - if they're not set just add some defaults
 	for configKey, defaultValue := range map[string]string{"user.name": "Gitea", "user.email": "gitea@fake.local"} {
 		if err := checkAndSetConfig(configKey, defaultValue, false); err != nil {