BUG: fix origin epoch when freq is Day and harmonize epoch between timezones
Follow-up of #31809.

The purpose of this PR is to fix the current behavior on master:

```python
import pandas as pd
import numpy as np

start, end = "2000-08-02 23:30:00+0500", "2000-12-02 00:30:00+0500"
rng = pd.date_range(start, end, freq="7min")
ts = pd.Series(np.random.randn(len(rng)), index=rng)

result_1 = ts.resample("D", origin="epoch").count()
result_2 = ts.resample("24H", origin="epoch").count()

print(f"result_1:\n\n{result_1}\n")
print(f"result_2:\n\n{result_2}\n")
```

Outputs on master:
```
result_1:

2000-08-02 00:00:00+05:00      5
2000-08-03 00:00:00+05:00    205
2000-08-04 00:00:00+05:00    206
2000-08-05 00:00:00+05:00    206
2000-08-06 00:00:00+05:00    206
                            ... 
2000-11-28 00:00:00+05:00    206
2000-11-29 00:00:00+05:00    206
2000-11-30 00:00:00+05:00    205
2000-12-01 00:00:00+05:00    206
2000-12-02 00:00:00+05:00      5
Freq: D, Length: 123, dtype: int64

result_2:

2000-08-02 05:00:00+05:00     48
2000-08-03 05:00:00+05:00    205
2000-08-04 05:00:00+05:00    206
2000-08-05 05:00:00+05:00    206
2000-08-06 05:00:00+05:00    205
                            ... 
2000-11-27 05:00:00+05:00    206
2000-11-28 05:00:00+05:00    206
2000-11-29 05:00:00+05:00    206
2000-11-30 05:00:00+05:00    205
2000-12-01 05:00:00+05:00    168
Freq: 24H, Length: 122, dtype: int64
```

Expected Outputs (with this PR):
```
result_1:

2000-08-02 00:00:00+05:00      5
2000-08-03 00:00:00+05:00    205
2000-08-04 00:00:00+05:00    206
2000-08-05 00:00:00+05:00    206
2000-08-06 00:00:00+05:00    206
                            ... 
2000-11-28 00:00:00+05:00    206
2000-11-29 00:00:00+05:00    206
2000-11-30 00:00:00+05:00    205
2000-12-01 00:00:00+05:00    206
2000-12-02 00:00:00+05:00      5
Freq: D, Length: 123, dtype: int64

result_2:

2000-08-02 00:00:00+05:00      5
2000-08-03 00:00:00+05:00    205
2000-08-04 00:00:00+05:00    206
2000-08-05 00:00:00+05:00    206
2000-08-06 00:00:00+05:00    206
                            ... 
2000-11-28 00:00:00+05:00    206
2000-11-29 00:00:00+05:00    206
2000-11-30 00:00:00+05:00    205
2000-12-01 00:00:00+05:00    206
2000-12-02 00:00:00+05:00      5
Freq: 24H, Length: 123, dtype: int64
```

------

I thought of two possible solutions while fixing that:
1. Consider 'epoch' as a UNIX epoch: `pd.Timestamp(0, tz=index_tz)`
2. Consider that 'epoch' is just an helper to fix the origin: `pd.Timestamp("1970-01-01", tz=index_tz)`

To have similar results between timezones, I thought the solution 2 was a better choice. The solution 2 could also help us to set the behavior `origin=epoch` as the default one in a future version since the results would be quite similar with `origin=start_day` (that is not quite the case with a DST timezone, I can provide further details if needed on why this is the case).

(The solution 1 is correct in theory but is giving results that could be hard to explain to the end user.)

------

- [X] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry (follow-up PR, not necessary I think)
