use tag instead of commit hash

Author: PavelSafronov

.evergreen/config.in.yml

@@ -1298,6 +1298,6 @@ buildvariants:
     display_name: BSON compatibility tests
     run_on: rhel80-large
     expansions:
-      source_rev: 2512137b5d94b238e7cf5ee753f867e8d1e2e0fc
+      source_rev: refs/tags/v7.0.0
     tasks:
       - test-bson-latest-server-7.0.0

.evergreen/config.yml

@@ -3443,7 +3443,7 @@ buildvariants:
     display_name: BSON compatibility tests
     run_on: rhel80-large
     expansions:
-      source_rev: 2512137b5d94b238e7cf5ee753f867e8d1e2e0fc
+      source_rev: refs/tags/v7.0.0
     tasks:
       - test-bson-latest-server-7.0.0
   - name: rhel80-large-iron