I will check that MySQL, PostgreSQL and H2 supports column change to TEXT from VARCHAR. Please merge after that.

Oops. It needs an index for TEXT column but it is not supported. Any ideas?
A webhook URL should be longer than 255 and cannot be VARCHAR.

I have two ideas:

• Modify (or remove) constraints and keep consistency by application layer
• Add alternative columns which store hash value of text columns, and use these columns for constraints

I think the second one is a right way but it needs complex migration.

I exactly dont'n know about H2 and pgsql, but mysql max varchar is 65,535 and max row length 65,535 bytes too The CHAR and VARCHAR Types, Row Size Limits:
H2 Limits and Limitations

There is no limit for the following entities, except the memory and storage capacity: maximum identifier length (table name, column name, and so on); maximum number of tables, columns, indexes, triggers, and other database objects; maximum statement length, number of parameters per statement, tables per statement, expressions in order by, group by, having, and so on; maximum rows per query; maximum columns per table, columns per index, indexes per table, lob columns per table, and so on; maximum row length, index row length, select row length; maximum length of a varchar column, decimal column, literal in a statement.

pgsql Character Types:

In any case, the longest possible character string that can be stored is about 1 GB

There is problem with max key length (in mysql 1024bytes, i think), but why indexing URL column? maybe you have reason for unique url per repository? (except noob users :) ) maybe better only index USER_NAME + REPOSITORY_NAME for webhook search when run it? little index = fast search
edit
for foreign key constraints to WEB_HOOK_EVENT set id (autoincrement, bigint) and use join for select