Backport to 2.17.x: #7456: Fix regression test output due to upstream changes

[timescale-automation]

This is an automated backport of #7456: Fix regression test output due to upstream changes. This PR will be merged automatically after all the relevant CI checks pass. If this fix should not be backported, or will be backported manually, just close this PR. You can use the backport branch to add your changes, it won't be modified automatically anymore.

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Original description

Fix regression test output due to upstream changes

Recently Postgres updated time zone data files to tzdata2024b:

• 17.1: https://github.com/postgres/postgres/commit/cad65907ee
• 16.5: https://github.com/postgres/postgres/commit/a0c8d600bb
• 15.9: https://github.com/postgres/postgres/commit/74f70cb8633
• 14.14: https://github.com/postgres/postgres/commit/dedced73e00

In #7276 we forced our regression tests to explicit use PST8PDT timezone but for some reason on tzdata2024b it definition changed. The change is valid for dates before 1970 and this is exactly the case here for the CAgg watermark test outputs.

We're also noticing consistently failures in our CI:

• https://github.com/timescale/timescaledb/actions/runs/11883366631/job/33110029493#step:20:35
• https://github.com/timescale/timescaledb/actions/runs/11883366631/job/33110029375#step:20:35

Fixed it by using the timezone UTC-8 instead to keep compatibility with all calculations in the regression suite. The only noticible change is timezone output information.

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