Auto merge of #28654 - wusyong:bootstrap, r=jdm

Allow bootstrap on Pop!_OS 21.10

<!-- Please describe your changes on the following line: -->
bootstrap.py doesn't support Pop!_OS 21.x yet. Adding these lines should make the script works again.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] These changes do not require tests because it seems there no test requires certain version check and test-tidy has passed.

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python/servo/bootstrap.py

@@ -355,7 +355,9 @@ def get_linux_distribution():
         else:
             major = version
 
-        if major == '20':
+        if major == '21':
+            base_version = '21.04'
+        elif major == '20':
             base_version = '20.04'
         elif major == '19':
             base_version = '18.04'