unify keyword to uppercase

[shawn0915]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

https://ask.pingcap.com/t/keyword-uppercase-or-lowercase/10266

keyword:

• TIDB
• TiDB_CURRENT_TSO (R)

shall we unify the keyword to uppercase?

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)