build: modernize Go usage

This commit modernizes Go usage. This was done with:

go run golang.org/x/tools/gopls/internal/analysis/modernize/cmd/modernize@latest -fix -test ./...

Then files needed to be `go fmt`ed and a few comments needed to be
restored.

The modernizations include replacing

- if/else conditional assignment by a call to the built-in min or max functions added in go1.21
- sort.Slice(x, func(i, j int) bool) { return s[i] < s[j] } by a call to slices.Sort(s), added in go1.21
- interface{} by the 'any' type added in go1.18
- append([]T(nil), s...) by slices.Clone(s) or slices.Concat(s), added in go1.21
- loop around an m[k]=v map update by a call to one of the Collect, Copy, Clone, or Insert functions from the maps package, added in go1.21
- []byte(fmt.Sprintf...) by fmt.Appendf(nil, ...), added in go1.19
- append(s[:i], s[i+1]...) by slices.Delete(s, i, i+1), added in go1.21
- a 3-clause for i := 0; i < n; i++ {} loop by for i := range n {}, added in go1.22

backend/jottacloud/api/types.go

@@ -70,7 +70,7 @@ func (t *Rfc3339Time) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 
 // MarshalJSON turns a Rfc3339Time into JSON
 func (t *Rfc3339Time) MarshalJSON() ([]byte, error) {
-	return []byte(fmt.Sprintf("\"%s\"", t.String())), nil
+	return fmt.Appendf(nil, "\"%s\"", t.String()), nil
 }
 
 // LoginToken is struct representing the login token generated in the WebUI
@@ -165,25 +165,25 @@ type DeviceRegistrationResponse struct {
 
 // CustomerInfo provides general information about the account. Required for finding the correct internal username.
 type CustomerInfo struct {
-	Username          string      `json:"username"`
-	Email             string      `json:"email"`
-	Name              string      `json:"name"`
-	CountryCode       string      `json:"country_code"`
-	LanguageCode      string      `json:"language_code"`
-	CustomerGroupCode string      `json:"customer_group_code"`
-	BrandCode         string      `json:"brand_code"`
-	AccountType       string      `json:"account_type"`
-	SubscriptionType  string      `json:"subscription_type"`
-	Usage             int64       `json:"usage"`
-	Quota             int64       `json:"quota"`
-	BusinessUsage     int64       `json:"business_usage"`
-	BusinessQuota     int64       `json:"business_quota"`
-	WriteLocked       bool        `json:"write_locked"`
-	ReadLocked        bool        `json:"read_locked"`
-	LockedCause       interface{} `json:"locked_cause"`
-	WebHash           string      `json:"web_hash"`
-	AndroidHash       string      `json:"android_hash"`
-	IOSHash           string      `json:"ios_hash"`
+	Username          string `json:"username"`
+	Email             string `json:"email"`
+	Name              string `json:"name"`
+	CountryCode       string `json:"country_code"`
+	LanguageCode      string `json:"language_code"`
+	CustomerGroupCode string `json:"customer_group_code"`
+	BrandCode         string `json:"brand_code"`
+	AccountType       string `json:"account_type"`
+	SubscriptionType  string `json:"subscription_type"`
+	Usage             int64  `json:"usage"`
+	Quota             int64  `json:"quota"`
+	BusinessUsage     int64  `json:"business_usage"`
+	BusinessQuota     int64  `json:"business_quota"`
+	WriteLocked       bool   `json:"write_locked"`
+	ReadLocked        bool   `json:"read_locked"`
+	LockedCause       any    `json:"locked_cause"`
+	WebHash           string `json:"web_hash"`
+	AndroidHash       string `json:"android_hash"`
+	IOSHash           string `json:"ios_hash"`
 }
 
 // TrashResponse is returned when emptying the Trash