Revert "MM-54502 - Update regex to force first character to be alpha … (#28860)

* Revert "MM-54502 - Update regex to force first character to be alpha (#24675)"

This reverts commit 08ed72f060.

* update tests for reverted PR

* Revert "update tests for reverted PR"

This reverts commit c1cf5495fa129008e44a8fcdc0ee34762b33dd34.

* Revert "Revert "MM-54502 - Update regex to force first character to be alpha (#24675)""

This reverts commit 6920b979c8dba86a837bafff7c057dc609c6aa58.

* only revert important parts, not tests

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/user_test.go

@@ -431,9 +431,9 @@ var usernames = []usernamesTest{
 	{"spin-punch", true, true},
 	{"sp", true, true},
 	{"s", true, true},
-	{"1spin-punch", false, false},
-	{"-spin-punch", false, false},
-	{".spin-punch", false, false},
+	{"1spin-punch", true, true},
+	{"-spin-punch", true, true},
+	{".spin-punch", true, true},
 	{"Spin-punch", false, false},
 	{"spin punch-", false, false},
 	{"spin_punch", true, true},