Fix links in templates, gcode.h (#14160)

.github/issue_template.md

@@ -1,6 +1,6 @@
 <!--
 
-Have you read Marlin's Code of Conduct? By filing an Issue, you are expected to comply with it, including treating everyone with respect: https://github.com/MarlinFirmware/Marlin/blob/master/.github/code_of_conduct.md
+Have you read Marlin's Code of Conduct? By filing an Issue, you are expected to comply with it, including treating everyone with respect: https://github.com/MarlinFirmware/Marlin/blob/bugfix-2.0.x/.github/code_of_conduct.md
 
 Do you want to ask a question? Are you looking for support? Please don't post here. Instead please use the Marlin Firmware forum at http://forums.reprap.org/list.php?415 or the Marlin Facebook Group https://www.facebook.com/groups/1049718498464482/.
 

Marlin/src/gcode/gcode.h

@@ -30,11 +30,11 @@
  * -----------------
  *
  * Helpful G-code references:
- *  - http://linuxcnc.org/handbook/gcode/g-code.html
+ *  - http://marlinfw.org/meta/gcode
- *  - http://objects.reprap.org/wiki/Mendel_User_Manual:_RepRapGCodes
+ *  - https://reprap.org/wiki/G-code
+ *  - http://linuxcnc.org/docs/html/gcode.html
  *
  * Help to document Marlin's G-codes online:
- *  - http://reprap.org/wiki/G-code
  *  - https://github.com/MarlinFirmware/MarlinDocumentation
  *
  * -----------------