Increase string attribute max length to 256 chars #892

Job	Run time
test (py39, postgresql-file-upgrade-from-4.5)	1s
doc (docs)	1m 38s
check (build)	40s
test (py39, mysql-file)	54s
doc (docs-gnocchi-web)	1m 33s
check (pep8)	43s
test (py39, mysql-file-sqlalchemy14)	1s
test (py39, mysql-swift)	1s
test (py39, mysql-s3)	1s
test (py39, postgresql-file)	1s
test (py39, postgresql-file-sqlalchemy14)	1s
test (py39, postgresql-swift)	1s
test (py39, postgresql-s3)	1s
test (py311, postgresql-file-upgrade-from-4.5)	2m 39s
test (py311, mysql-file)	2m 40s
test (py311, mysql-file-sqlalchemy14)	2m 37s
test (py311, mysql-swift)	2m 37s
test (py311, mysql-s3)	2m 37s
test (py311, postgresql-file)	2m 23s
test (py311, postgresql-file-sqlalchemy14)	2m 22s
test (py311, postgresql-swift)	2m 33s
test (py311, postgresql-s3)	2m 37s
test (py312, mysql-ceph-upgrade-from-4.5)	2m 39s
test (py312, postgresql-file-upgrade-from-4.5)	1m 48s
test (py312, mysql-file)	2m 37s
test (py312, mysql-file-sqlalchemy14)	2m 38s
test (py312, mysql-swift)	2m 37s
test (py312, mysql-s3)	2m 38s
test (py312, postgresql-file)	2m 27s
test (py312, postgresql-file-sqlalchemy14)	2m 37s
test (py312, postgresql-swift)	54s
test (py312, postgresql-s3)	59s
	50m 35s

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