update cosmos log color to purple (#494)

## Description

The previous color was the same as error messages and confusing to read
the logs. Updating to purple.

## Related Issue(s)

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## Breaking Change?

no, purely cosmetic.

## Checklist

- [ ] I have made corresponding changes to the documentation (if
required)
- [ ] I have added tests that prove my fix is effective or that my
feature works

cosmos/log.py

@@ -8,7 +8,7 @@
     "[%(blue)s%(asctime)s%(reset)s] "
     "{%(blue)s%(filename)s:%(reset)s%(lineno)d} "
     "%(log_color)s%(levelname)s%(reset)s - "
-    "%(yellow)s(astronomer-cosmos)%(reset)s - "
+    "%(purple)s(astronomer-cosmos)%(reset)s - "
     "%(log_color)s%(message)s%(reset)s"
 )
 

tests/test_log.py

@@ -4,7 +4,7 @@
 
 
 def test_get_logger():
-    custom_string = "%(yellow)s(astronomer-cosmos)%(reset)s"
+    custom_string = "%(purple)s(astronomer-cosmos)%(reset)s"
     standard_logger = logging.getLogger()
     assert custom_string not in standard_logger.handlers[0].formatter._fmt