[Fixes #14673]: Upload fails with duplicate key value violates unique constraint when column names start with a digit

[mwallschlaeger]

Description

Prefixed the name with _ instead of substituting inside it, and do it after slugify so the underscore isn't stripped:

name = slugify.slugify(name[:62], separator="_")

if name[:1].isdigit():
    name = f"_{name}"

3_1a → _3_1a, 6_1a → _6_1a: distinct names, nothing deleted. The function is now also idempotent means under certain cirumstances it is currently possible that this function get called multiple times. Result now stays the same

Behaviour change

fixup_name() is used for layer/table names as well, so a layer named 1layername now normalises to _1layername instead of all appearances of the the first digit in a layername get replaced by _.

closes #14673

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