[FLINK-34067] Fix javacc warnings in flink-sql-parser

[jnh5y]

What is the purpose of the change

Address the two warnings in the build for the Flink SQL Parser:

[INFO] — javacc:2.4:javacc (javacc) @ flink-sql-parser ---                                                 
Java Compiler Compiler Version 4.0 (Parser Generator)                                                       
(type "javacc" with no arguments for help)                                                                                                 
Reading from file .../flink-table/flink-sql-parser/target/generated-sources/javacc/Parser.jj . . .               
Note: UNICODE_INPUT option is specified. Please make sure you create the parser/lexer using a Reader with the correct character encoding.  
Warning: Choice conflict involving two expansions at                                                           
         line 2043, column 13 and line 2052, column 9 respectively.                                        
         A common prefix is: "IF"        
        Consider using a lookahead of 2 for earlier expansion.                                                
Warning: Choice conflict involving two expansions at                                                          
         line 2097, column 13 and line 2105, column 8 respectively.                                         
         A common prefix is: "IF"                                                                                                  
         Consider using a lookahead of 2 for earlier expansion.     

Brief change log

Adds LOOKAHEAD(2) to the necessary places in parserImpls.ftl.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage. One can verify the change by building with mvn -Pfast -f flink-table/flink-sql-parser install

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• 319b84aca99ca02d59dbfb7139a92472d0b19fd3 Azure: SUCCESS

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[jnh5y]

@flinkbot run azure