Update dependency com.thoughtworks.xstream:xstream to v1.4.18

[mend-for-github-com[bot]]

This PR contains the following updates:

Package	Type	Update	Change
com.thoughtworks.xstream:xstream (source)	compile	patch	1.4.5 -> 1.4.18

By merging this PR, the issue #6 will be automatically resolved and closed:

Severity	CVSS Score	CVE
High	9.9	CVE-2021-21345
High	9.8	CVE-2013-7285
High	9.8	CVE-2021-21344
High	9.8	CVE-2021-21346
High	9.8	CVE-2021-21347
High	9.8	CVE-2021-21350
High	9.1	CVE-2021-21342
High	9.1	CVE-2021-21351
High	8.8	CVE-2020-26217
High	8.8	CVE-2021-29505
High	8.8	CVE-2021-39139
High	8.6	CVE-2021-21349
High	8.5	CVE-2021-39141
High	8.5	CVE-2021-39144
High	8.5	CVE-2021-39145
High	8.5	CVE-2021-39146
High	8.5	CVE-2021-39147
High	8.5	CVE-2021-39148
High	8.5	CVE-2021-39149
High	8.5	CVE-2021-39150
High	8.5	CVE-2021-39151
High	8.5	CVE-2021-39152
High	8.5	CVE-2021-39153
High	8.5	CVE-2021-39154
High	7.7	CVE-2020-26258
High	7.5	CVE-2016-3674
High	7.5	CVE-2017-7957
High	7.5	CVE-2021-21341
High	7.5	CVE-2021-21343
High	7.5	CVE-2021-21348
High	7.5	CVE-2021-43859
Medium	6.8	CVE-2020-26259
Medium	6.3	CVE-2021-39140

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