fix #99 Fix the migrations on PostgreSQL and document its support

[njoubert-cleverage]

Description

Fixes #99.

No project runs the bundle on PostgreSQL: none of its migrations has been executed on PostgreSQL, so their PostgreSQL branches are fixed in place (no new migration):

• Version20231006111525: the id columns of log_record, process_execution and process_user are created as identity columns (id INT GENERATED BY DEFAULT AS IDENTITY, as generated by Doctrine). The standalone <table>_id_seq sequences were not used as default value: inserts without id (IDENTITY generation, the doctrine/orm 3 default) failed with null value in column "id".
• Version20240729151928: process_schedule created with the PostgreSQL syntax (was AUTO_INCREMENT: syntax error, the migrations stopped there).
• Docs: docs/index.md mentions the supported databases (MySQL / MariaDB and PostgreSQL).

process_execution.context (NOT NULL in the migrations, nullable in the mapping) is a mapping / schema difference, not a PostgreSQL issue (ADD ... NOT NULL works on the empty table of a fresh installation): handled with #97.

Tests of the migrations removed (tests/Migrations/, separate commit): they only asserted the generated SQL, never executed on a database, and did not catch these PostgreSQL failures.

Checked with the bundle migrations run by doctrine/migrations, then SchemaTool and ORM inserts (doctrine/orm 3.7, DBAL 4.5):

• PostgreSQL 16, fresh database: migrations OK; schema diff: only the cosmetic (DC2Type:...) column comments; ORM inserts of User, ProcessExecution, LogRecord, ProcessSchedule OK. Before this PR: syntax error at or near "AUTO_INCREMENT"
• PostgreSQL 16, down to the first migration then up: OK, same schema, inserts OK
• MySQL 9.1, fresh database: migrations OK, schema diff: only the (DC2Type:...) comments
• process-bundle-demo on PostgreSQL 16 (cleverage/ui-process-bundle#99 - Allow running the demo on PostgreSQL instead of MySQL process-bundle-demo#70): make start migrations and fixtures, doctrine demo processes, UI login, launch from the UI consumed by the worker, schedule creation
• bundle (Symfony 8.1, PHP 8.5): 257 tests OK (330 before the removal of the migration tests). PHPStan, PHP-CS-Fixer, Rector OK

Not handled here: running the migrations on PostgreSQL / MySQL in the CI.

Requirements

• Documentation updates
  • Reference
  • Changelog
• Unit tests

Breaking changes

None: the PostgreSQL migrations failed, no project runs on PostgreSQL.

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