WT-12793 Remove 6.0 tests after removing support for downgrading data…

…base files beyond 7.0 (#10466)

The main change involves updating the default compatibility
configuration settings in the Python compatibility testing framework so
that we no longer test on versions below MongoDB 6.0. The other changes
involve removing tests which are no longer needed.

The fast-truncate related tests in the Python compatibility suite were
initially used to test whether version 6.0 could read database files
generated by newer versions correctly, and were added to allow for
automated upgrade/downgrade testing. Now that we have bumped up the
`WT_MIN_STARTUP_VERSION` minimum value, we don't officially support
downgrading database files beyond version 7.0. Consequently, these tests
are no longer relevant.

test/compatibility/common/compatibility_config.py

@@ -31,10 +31,10 @@
 # The branches we use for the testing. We support special branch name 'this' that refers to the
 # current branch. This is useful when debugging a compatibility issue on the current branch, but it
 # should not be enabled when testing on Evergreen.
-BRANCHES = ['develop', 'mongodb-7.1', 'mongodb-7.0', 'mongodb-6.3', 'mongodb-6.0']
+BRANCHES = ['develop', 'mongodb-7.1', 'mongodb-7.0', 'mongodb-6.3']
 
 # Example use of the 'this' branch (useful for debugging):
-# BRANCHES = ['this', 'mongodb-6.0']
+# BRANCHES = ['this', 'mongodb-7.0']
 
 # The default directory to which the test will check out other branches, relative to the project's
 # top-level directory.