Add 6.18, 6.19 and 6.20 to check_format_compatible.sh (#8236)

Summary:
Add 6.18, 6.19 and 6.20 to check_format_compatible.sh

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8236

Test Plan: ./tools/check_format_compatible.sh (tested without 2.7.fb as it was failing as mentioned in the script)

Reviewed By: mrambacher

Differential Revision: D28019160

Pulled By: akankshamahajan15

fbshipit-source-id: b59a7c5c14cb4c115926e9ae7c74ea586b22c9ed

tools/check_format_compatible.sh

@@ -117,7 +117,7 @@ EOF
 
 # To check for DB forward compatibility with loading options (old version
 # reading data from new), as well as backward compatibility
-declare -a db_forward_with_options_refs=("6.6.fb" "6.7.fb" "6.8.fb" "6.9.fb" "6.10.fb" "6.11.fb" "6.12.fb" "6.13.fb" "6.14.fb" "6.15.fb" "6.16.fb" "6.17.fb")
+declare -a db_forward_with_options_refs=("6.6.fb" "6.7.fb" "6.8.fb" "6.9.fb" "6.10.fb" "6.11.fb" "6.12.fb" "6.13.fb" "6.14.fb" "6.15.fb" "6.16.fb" "6.17.fb" "6.18.fb" "6.19.fb" "6.20.fb")
 # To check for DB forward compatibility without loading options (in addition
 # to the "with loading options" set), as well as backward compatibility
 declare -a db_forward_no_options_refs=() # N/A at the moment