Fix BigQuery jobs Label Regex

[pocky13]

#37736 Add label DAG ID and Task ID to BigQuery jobs.
However, the maximum label length is 63 characters (see: https://cloud.google.com/bigquery/docs/labels-intro#requirements). Therefore, if the DAG ID or Task ID length is 64, this causes an invalid label exception.
This PR fixes that issue.

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[eladkal]

Can you add unit test to avoid regression?

[eladkal]

Another PR that try to solve the same bug
#39568

[shahar1]

@pocky13 this is actually the more precise fix for now, as the docs state:

label keys and values can be no longer than 63 characters, can only contain lowercase letters, numeric characters, underscores and dashes. International characters are allowed. Label values are optional. Label keys must start with a letter and each label in the list must have a different key. 

In #39568, the regex currently looks like this
LABEL_REGEX = re.compile(r"^[\w-]{0,63}$")
which lets the first character to be an invalid character.

Edit: valid only for the key as explained in the 2nd PR.

[eladkal]

Thank you for your contribution.
#39568 solved the bug