fix: update samples snippet to write to BYTES instead of ARRAY<BYTES>

[forksumit]

Update samples snippet InsertingDataTypes.java

• Change datatype for school column from REPEATED BYTES to BYTES
• Convert byte array to base64 string when writing into BYTES column (school)

Fixes #2875 ☕️

[obada-ab]

LGTM, manually tested and after the change a correct base64 encoded string is being uploaded to BQ

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