Update postgresqlreceiver vacuum metric unit to vacuum not vacuums

[beshbeeshy]

Updating the unit of the metric postgresql.table.vacuum.count in postgresqlreceiver to be vacuum instead of vacuums, to comply with the semantic conventions.

Link to tracking issue

Fixes: #43207

Testing

Documentation

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[dmitryax]

Please add a changelog item in .chlogen

[beshbeeshy]

@dmitryax Absolutely! Thanks.

[beshbeeshy]

@dmitryax Would this be considered a breaking change? I made it non-breaking because the unit is actually the same with the same semantics except for the name now is using the singular form.

WDYT?

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