Increase string attribute max length to 256 chars #893

Job	Run time
doc (docs)	1m 50s
check (build)	38s
doc (docs-gnocchi-web)	1m 33s
check (pep8)	44s
test (py39, postgresql-file-upgrade-from-4.5)	2m 58s
test (py39, mysql-file)	3m 0s
test (py39, mysql-file-sqlalchemy14)	2m 59s
test (py39, mysql-swift)	3m 44s
test (py39, mysql-s3)	4m 1s
test (py39, postgresql-file)	2m 48s
test (py39, postgresql-file-sqlalchemy14)	2m 46s
test (py39, postgresql-swift)	3m 22s
test (py39, postgresql-s3)	3m 30s
test (py311, postgresql-file-upgrade-from-4.5)	2m 39s
test (py311, mysql-file)	2m 50s
test (py311, mysql-file-sqlalchemy14)	3m 1s
test (py311, mysql-swift)	3m 29s
test (py311, mysql-s3)	3m 57s
test (py311, postgresql-file)	2m 50s
test (py311, postgresql-file-sqlalchemy14)	2m 45s
test (py311, postgresql-swift)	3m 3s
test (py311, postgresql-s3)	3m 20s
test (py312, mysql-ceph-upgrade-from-4.5)	2m 35s
test (py312, postgresql-file-upgrade-from-4.5)	2m 33s
test (py312, mysql-file)	3m 2s
test (py312, mysql-file-sqlalchemy14)	2m 56s
test (py312, mysql-swift)	3m 36s
test (py312, mysql-s3)	3m 46s
test (py312, postgresql-file)	2m 43s
test (py312, postgresql-file-sqlalchemy14)	2m 41s
test (py312, postgresql-swift)	3m 13s
test (py312, postgresql-s3)	3m 28s
	1h 32m 20s

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