pgsql: Rename the role in stats_ext to have regress_ prefix

Rename the role in stats_ext to have regress_ prefix

Commit 5be8ce82e8 added a new role to the stats_ext regression suite,
but the role name did not start with regress_ causing failures when
running with ENFORCE_REGRESSION_TEST_NAME_RESTRICTIONS. Fixed by
renaming the role to start with the expected regress_ prefix.

Backpatch-through: 10, same as the new regression test
Discussion: https://postgr.es/m/1F238937-7CC2-4703-A1B1-6DC225B8978A%40enterprisedb.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/5f8dd5dc1701348bce659b9689d085e011399d61

Modified Files
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src/test/regress/expected/stats_ext.out | 6 +++---
src/test/regress/sql/stats_ext.sql      | 6 +++---
2 files changed, 6 insertions(+), 6 deletions(-)