Add LastDayOfMonth database function

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

Add a built-in `LastDayOfMonth` database function that returns the last calendar day of the month for a given `DateField` or `DateTimeField` expression.

### Problem

Today developers must hand-roll backend-specific `Func` subclasses (or wrap raw SQL) to calculate “last day of month”. That works but it’s verbose, copied across code-bases and easy to get subtly wrong (leap years, Oracle quirks, new DB versions). A small built-in helper removes that friction and raises the quality bar.

Why a core helper is better than ad-hoc `Func` / `FuncWrapper`:

* **One-liner, zero boiler-plate** – `LastDayOfMonth("start")`.
* **Cross-backend guarantee** – ORM ships the right SQL for PostgreSQL, MySQL/MariaDB, SQLite and Oracle, covered by the test-suite.
* **Official docs & IDE discoverability** – lives next to `TruncMonth`, `ExtractDay`, etc.
* **Clearer code reviews** – intent obvious without opening a helper file; no naming drift (`MonthEnd`, `EndOfMonth`, …).
* **Safer upgrades** – Django can warn if backend syntax changes, while custom SQL might silently break.
* **Negligible maintenance cost** – MySQL/MariaDB and Oracle expose native `LAST_DAY()`, and the Postgres/SQLite expression is a single date-math line.

### Request or proposal

proposal

### Additional Details

#### Use-cases
* Payroll or invoice cut-off dates
* Month-end KPI / reporting roll-ups
* Any query that needs “<= last day of the month”

#### Proposed API
from django.db.models.functions import LastDayOfMonth
qs.annotate(period_end=LastDayOfMonth("issued_at"))

#### Backend SQL mapping (illustrative)
| Backend | SQL emitted |
| -------------- | ----------------------------------------------- |
| PostgreSQL | date_trunc('month', exp + interval '1 month') - interval '1 day' |
| MySQL/MariaDB | LAST_DAY(exp) |
| SQLite | date(exp, '+1 month', 'start of month', '-1 day') |
| Oracle | LAST_DAY(exp) (pure-date via TRUNC/NUMTODSINTERVAL) |

### Implementation Suggestions

* `class LastDayOfMonth(Func)` in _django/db/models/functions/datetime.py_
- `arity = 1`
- `output_field = models.DateField()`
- backend methods: `as_postgresql()`, `as_mysql()`, `as_sqlite()`, `as_oracle()`
* Tests: `tests/db_functions/datetime/test_last_day_of_month.py` using a simple model with a `DateField`.
* Docs: add to “Database Functions” reference and to the upcoming 5.2 release notes under “Minor features”.
* I’m happy to open the PR and iterate on review feedback.