Update dob from xth 2019 to xth 2021

Sample Data
====================
collection : cbse1019

{
"_id" : ObjectId("60e447d270753939035f143f"),
"REG" : "A",
"ROLL" : 1100001111,
"CNAME" : "AAMRITA BHATTACHARYYA",
"MNAME" : "RANITA BHATTACHARYYA",
"FNAME" : "ATANU BHATTACHARYYA",
"GENDER" : "F",
"DOB" : 1072003,
"MOBILE" : NumberLong(9979859444)
}

{
"_id" : ObjectId("60e447d270753939035f1440"),
"REG" : "A",
"ROLL" : 1100002112,
"CNAME" : "Sunil",
"MNAME" : "Sunil",
"FNAME" : "KALPESH SHETH",
"GENDER" : "F",
"DOB" : 12052002,
"MOBILE" : NumberLong(9327004366)
}

{
"_id" : ObjectId("60e447d270753939035f1440"),
"REG" : "A",
"ROLL" : 11618279113,
"CNAME" : "ANVI SHETH",
"MNAME" : "Sunil ",
"FNAME" : "Sunil",
"GENDER" : "F",
"DOB" : 12052002,
"MOBILE" : NumberLong(9327004366)
}


--------------------- cbse1221 ---------
{
"_id" : ObjectId("60e4539627b2d56c9a634de6"),
"REG" : "A",
"RROLL" : 1100001111,
"CNAME" : "AAMRITADD ",
"MNAME" : "RANITADD ",
"FNAME" : "ATANU ",
"GENDER" : "F",
"AADHAR" : NumberLong(7863168625yy),
"MOBILE" : NumberLong(9979859511),
"EMAIL" : "tukai70@rediffmail.com",
"CLASS" : 12,
"PREV_ROLL" : "",
"PREV_YEAR" : "",
"CLS10_ROLL" : 1100001111,
"CLS10_YR" : 2019,
"SEC_BOARD" : "CBSE",
"PPATH" : "photo/A/12/A221101730124"
}

{
"_id" : ObjectId("60e4539627b2d56c9a634de6"),
"REG" : "A",
"RROLL" : 1100002112
"CNAME" : "AAMRITASS ",
"MNAME" : "RANITAAA ",
"FNAME" : "ATANU ",
"GENDER" : "F",
"AADHAR" : NumberLong(7863168625yy),
"MOBILE" : NumberLong(9979859511),
"EMAIL" : "tukai70@rediffmail.com",
"CLASS" : 12,
"PREV_ROLL" : "",
"PREV_YEAR" : "",
"CLS10_ROLL" : 1100002112,
"CLS10_YR" : 2019,
"SEC_BOARD" : "CBSE",
"PPATH" : "photo/A/12/A221101730124"
}
{
"_id" : ObjectId("60e4539627b2d56c9a634de6"),
"REG" : "A",
"RROLL" : 11618279113,
"CNAME" : "AAMRITA kkk ",
"MNAME" : "RArNITA ",
"FNAME" : "ATANeU ",
"GENDER" : "F",
"AADHAR" : NumberLong(7863168625yy),
"MOBILE" : NumberLong(9979859511),
"EMAIL" : "tukai70@rediffmail.com",
"CLASS" : 12,
"PREV_ROLL" : "",
"PREV_YEAR" : "",
"CLS10_ROLL" : 11618279113,
"CLS10_YR" : 2019,
"SEC_BOARD" : "CBSE",
"PPATH" : "photo/A/12/A221101730124"
}