pgsql: ci: Make compute resources for CI configurable

ci: Make compute resources for CI configurable

See prior commit for an explanation for the goal of the change and why it had
to be split into two commits.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/20230808021541.7lbzdefvma7qmn3w@awork3.anarazel.de
Backpatch: 15-, where CI support was added

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e8a8cd05d4b9533cf5bff0f4a4ee83a40df62875

Modified Files
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.cirrus.tasks.yml   | 45 ---------------------------------
.cirrus.yml         | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++
src/tools/ci/README | 17 +++++++++++++
3 files changed, 90 insertions(+), 45 deletions(-)