fix: use double-dash long options for syncthing v2.0.0

[ngc7331]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

In syncthing v2.0.0, they have dropped old single-dash long options. The -home used in the original startup script will be treated as -h -o -m -e, causing service start failure.

Modernised command line options parsing. Old single-dash long options are
no longer supported, e.g. -home must be given as --home. Some options
have been renamed, others have become subcommands. All serve options are
now also accepted as environment variables. See syncthing --help and
syncthing serve --help for details.

Benefits of this PR and context:

This PR changed old single-dash long options to double-dash ones.

How Has This Been Tested?

I've ran docker build -t ngc7331/syncthing --no-cache --pull . and docker run --name=syncthing-test -d -p 8385:8384 ngc7331/syncthing. Access through webui and it works well for me.

Source / References:

• https://github.com/syncthing/syncthing/releases/tag/v2.0.0

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[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/syncthing/v2.0.0-pkg-7c7a51df-dev-3e5938c20c158c73cb56f7427f3203662b59d684-pr-94/index.html
https://ci-tests.linuxserver.io/lspipepr/syncthing/v2.0.0-pkg-7c7a51df-dev-3e5938c20c158c73cb56f7427f3203662b59d684-pr-94/shellcheck-result.xml

Tag	Passed
amd64-v2.0.0-pkg-7c7a51df-dev-3e5938c20c158c73cb56f7427f3203662b59d684-pr-94	✅
arm64v8-v2.0.0-pkg-7c7a51df-dev-3e5938c20c158c73cb56f7427f3203662b59d684-pr-94	✅