Bump jmustache 1.15 -> 1.16

[bachorp]

Superseeds https://github.com/OpenAPITools/openapi-generator/pull/19575, resolves https://github.com/OpenAPITools/openapi-generator/issues/20375.

I applied the following logic to adjust our templates to jmustache's changed semantics.

import re
from pathlib import Path

for file in filter(
    lambda p: p.is_file()
    and not p.name.endswith(".jar.mustache")
    and not p.name.endswith(".war.mustache"),
    Path("modules/openapi-generator/src/main/resources").glob("**/*.mustache"),
):
    # I don't know how jmustache behaves with trailing whitespace, but this does not occur.
    assert not re.match(r"^ *\{\{>[^\}]+\}\} +$",
                        file.read_text(), flags=re.MULTILINE)
    file.write_text(
        # add comment to avoid standalone mode
        re.sub(
            r"^ +\{\{>[^\}]+\}\}$",
            lambda m: m.group(0) + "{{! prevent indent}}",
            # add newline to non-indented standalones (except at start of file)
            re.sub(
                r"^\{\{>[^\}]+\}\}\n",
                lambda m: m.group(0) if m.start(0) == 0 else m.group(0) + "\n",
                file.read_text(),
                flags=re.MULTILINE,
            ),
            flags=re.MULTILINE,
        )
    )

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