pgsql: Rename EXISTS-to-ANY converted subplan to exists_to_any

Rename EXISTS-to-ANY converted subplan to exists_to_any

Simple EXISTS subplans can be converted to hashed ANY subplans as
an alternative implementation. Previously, both the original EXISTS
subplan and the converted ANY subplan used names with the
exists_ prefix, making EXPLAIN output harder to read and
pg_plan_advice targets less clear.

Use the exists_to_any_ prefix for converted ANY subplans so that
their names distinguish them from the original EXISTS alternative.
This only changes the names shown by EXPLAIN and used by plan advice;
it does not affect planner behavior.

Author: Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Reviewed-by: solai v <solai(dot)cdac(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/20260605165641.3950f99ace0aad8f807abe96@sraoss.co.jp
Backpatch-through: 19

Branch
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3ab3f33281f62bdcef1dd987a89fcfe7f082b6d8

Modified Files
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contrib/pg_plan_advice/expected/alternatives.out | 30 ++++++++++++------------
contrib/pg_plan_advice/sql/alternatives.sql | 2 +-
src/backend/optimizer/plan/subselect.c | 8 +++++--
src/test/regress/expected/partition_prune.out | 10 ++++----
src/test/regress/expected/subselect.out | 16 ++++++-------
5 files changed, 35 insertions(+), 31 deletions(-)